Gemstone specifications
Gemstone quality
As a general rule, minerals and rocks classified as gemstones must share the important attributes of having visually attractive colours and textures, together with suitable durability so they may be cut and polished and thereby maintain that condition without being damaged excessively in wear or use.
In spite of this, comparatively softer materials are used as gemstones because their appeal is considered more important. Examples include variscite, a soft, attractive green phosphate mineral; opal, a brittle gemstone; and ‘zebra rock’, a siltstone with an attractive banded texture.
Stabilizing treatments can be applied to softer materials to prolong the quality of finish. In the past, an informal classification was developed to divide gems into precious and semiprecious groups largely based on their relative rarity. Historically, diamond, ruby, sapphire, emerald, precious opal, and alexandrite have been considered precious gems, with all others called semiprecious varieties. This grouping is no longer justified as each gemstone is assessed and appreciated on its individual merits, with its relative rarity determining its value.
Gemmological terminology
Most of the following chapters on gemstones and other ornamental materials contain highlighted boxes summarizing the gemmological properties of these materials.
The boxes vary with each type of gemstone and ornamental stone and show the important identifying parameters and characteristic features of each. Detailed references to these terms used can be found in many standard gemmological and geological books. Brief explanations of some of these terms are given below.
Appearance describes common colours, recognizable patterns, and textures characteristic of the material.
Birefringence, also called double refraction, is a measure of the difference between the lowest and highest refractive indices of an optically anisotropic gem mineral (a mineral with a physical property that varies with crystallographic direction). Minerals with high birefringence, such as zircon, may display images of their internal features that appear doubled in specific orientations. Double refraction is only possible in minerals that belong to non-isometric crystal systems and only when viewed in specific crystallographic directions. Minerals of the isometric (cubic) system are singly refractive and show no birefringence.
Cleavage and fracture are terms used to describe splitting or breakage of both minerals and rocks caused by planes or zones of weakness along which breakages may preferentially occur.
Fracture is an uneven breakage in any direction other than that produced by regular cleavage planes. A common example in rocks is conchoidal fracture, which results in concave depressions similar to bottle glass fracture. Fracture is seldom used as a diagnostic property of a mineral.
Mineral cleavage describes where a mineral breaks along one or more well-defined, crystallographic planes. These planar surfaces are described according to the nature of the surface, such as perfect, good, fair, or poor. Mineral cleavage is a physical property of a mineral relating to its atomic bonding that reflects its crystal structure. For example, diamond has perfect cleavage along planes parallel to its octahedral symmetry.
Cleavages defining planar surfaces in fine-grained rocks may have resulted from a number of effects including the formation of thin sheets parallel to folding pressures in slate and phyllite where the planar surfaces do not relate to former bedding planes. Alternatively, rock cleavage may result from a tendency to break along parallel layers of mineral concentrations, such as mica and quartz, that may be aligned to bedding planes or other preferred directions. Colour is an important guide in the identification of gem minerals and is a specific response of the eye to visible light (a range from 380 to 780 nm). Colour is perceived by the eye as a combination of both reflection and transmission of light as it interacts with a gem. The causes of colour in gems are complex. Some gems contain trace amounts of elements that generate colour change in an otherwise colourless gem, such as chromium and vanadium, that may result in green beryl known as emerald. The presence of trace elements affects the absorption of particular wavelengths in the gem and results in perceived colour differences. The majority of gem minerals, including quartz, diamond, beryl, tourmaline, topaz, and corundum, would be colourless (allochromatic) if they contained only the elements of their ideal chemical composition.
Other causes of colour include structural defects of the crystal lattice. Colour effects such as iridescence, where spectral colours are seen, result from diffraction and light interference effects caused by the physical structure of the material. Play-of-colour in precious opal and labradorescence displayed by some plagioclase feldspars results from the physical structures of these materials.
Crystal system refers to the symmetry group into which a mineral is classified based on its crystal structure. Each mineral can be classed into one of seven (previously six) crystal systems: isometric (cubic), tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic. Almost all minerals are crystalline. Non-crystalline minerals including glass and resins have no characteristic external form and are termed amorphous. An understanding of crystallography is important in gem identification and cutting as crystal structure influences many mineral properties.
Habit is the characteristic crystal form of a mineral, and may vary depending on the conditions of formation of the mineral. Habit is the easily observed expression of dominant crystal faces and is qualified by the appropriate form such as prismatic habit where describing columnarshaped crystals of, for example, beryl or tourmaline.
Hardness is stated as a number or range of numbers according to Mohs scale. The numbers denote an order of relative hardness or resistance to scratching and have no quantitative significance. Thus, a mineral’s hardness figure shows whether it can be scratched by a mineral harder than itself (i.e. it is of lower hardness), or whether it scratches a mineral of lower hardness on the scale (i.e. it is of greater hardness). The scale, first specified by the German mineralogist Friedrich Mohs, comprises 10 minerals in order of their comparative hardness from softest (1) to hardest (10):
Mohs hardness scale
1. talc
2. gypsum
3. calcite
4. fluorite
5. apatite
6. orthoclase feldspar
7. quartz
8. topaz
9. corundum
10. diamond.
Hardness test results establish some important distinctions between minerals of similar appearance such as beryl (hardness 7) and apatite (hardness 5). Hardness is a physical property that can be easily tested in the field on rough materials. Mineralogists also substitute other materials in the same way using a copper coin (hardness 3) or steel point (hardness 5.5)
Lustre is the brilliancy effect produced by light reflected from the surface of a gem. It is observed from a natural surface of a rock, mineral, or polished surface and its appearance is determined by the amount of incident light reflected from its surface. Gold and iron pyrite are both gem materials described as possessing a metallic lustre and many other gem minerals are described as having a vitreous (glass-like) lustre. Lustre is dependent on a combination of factors including the refractive index (RI) of the gem mineral and perfection of polish, which itself depends on the hardness of the material. Typically, minerals that are hard can be finished to a high polish and will retain the polish for longer because they are more resistant to abrasion.
Diamond has the greatest hardness of any natural mineral, high RI, and an adamantine lustre. This lustre type is also displayed by other gems with high RI such as zircon and demantoid garnet. Several rock types containing minerals of low hardness, such as serpentinites, have a waxy or resinous lustre.
Pleochroism is a general term used to describe change of colour shown by some coloured gems when viewed in different orientations. Pleochroism is most easily detected with the aid of a dichroscope, which allows two colours to be viewed side by side. Dichroic minerals show two different colours (a hue change) or different intensities of colour when viewed by a dichroscope in directions other than parallel to an optical axis. Trichroic minerals can display a maximum of three colours or three changes of colour.
Dichroic minerals belong to the tetragonal, hexagonal, and trigonal crystal systems, and trichroic minerals to the orthorhombic, monoclinic, and triclinic systems. Minerals of the isometric (cubic) system cannot display pleochroism.
Emerald and coloured tourmaline are examples of gem minerals that show strong dichroism. Cordierite (iolite) is an example of a trichroic gem showing three changes of colour from blue to violet-blue and yellow. Pleochroism (both dichroism and trichroism) can only be demonstrated in coloured gems.
Refractive index (RI) is shown as a number or range of numbers that express the optical density or refracting power of a mineral. For most minerals, refractive indices range from 1.35 to 2.42. RI is a ratio representing the velocity of light incident on a mineral (the velocity of light in air in practice is taken as a standard = 1.00) and its speed when transmitted through a mineral. RI is measured using a refractometer and standard models can detect a range of RI values between 1.4 and 1.8. In gem testing, an understanding of the behaviour of RI readings from a refractometer may be used to devise the optical character of a mineral. RI is probably the most important information used in gem mineral identification.
Specific gravity is the relative density of a material calculated as the mass or weight of a substance divided by the weight of an equal volume of pure water at 4°C.
Specific gravity is a ratio without units. It can be determined by different methods; gemmologists commonly use a specialized balance with dual pans that permit weighing of
the material in air and fully immersed in water. Calculation of specific gravity is derived using the formula: (weight of the material in air)/(weight of equivalent volume of water).
Transparency or diaphaneity expresses the degree to which light can be transmitted through a material.
Terms include grades of transparency ranging from fully transparent, where objects can be viewed through the mineral, to partially transparent as in semitransparent to translucent, where some light is transmitted. No light is transmitted through opaque materials such as gold. Most minerals and rocks have some degree of translucency in thin section.
Ultraviolet (UV) light and fluorescence. Ultraviolet light responses to gemstones are uncommon although photoluminescence reactions shown by some gem minerals may provide a useful indication of a gem’s identity.
Fluorescence is the emission of visible light by certain minerals when exposed to UV light.
Ultraviolet light, also known as black light (because it is invisible), is commonly categorized as short or long wave, based on the wavelength range produced by filters or mercury discharge lamps. Long-wave UV has a range from 315 to 400 nm, and short-wave UV from 200 to 280 nm.
Prospectors’ UV lamps are likely to have filters specifically made for detecting UV responses of certain economic minerals such as scheelite (calcium tungstate) and may emit a mix of long- and short-wave UV. Gem minerals that commonly respond to UV light include diamonds, topaz, and some synthetic gem materials such as cubic zirconia and glass (paste). Ultraviolet light will commonly cause surface crusts of opaline silica to fluoresce white or green, and calcite pink. Gemmology books provide reference tables on UV responses and information on personal safety for using UV light.
[
"Gemstone specifications Gemstone quality As a general rule, minerals and rocks classified as <COMMODITY>gemstones</COMMODITY> must share the important attributes of having visually attractive colours and textures, together with suitable durability so they may be cut and polished and thereby maintain that condition without being damaged excessively in wear or use",
" In spite of this, comparatively softer materials are used as <COMMODITY>gemstones</COMMODITY> because their appeal is considered more important",
" Examples include <COMMODITY>variscite</COMMODITY>, a soft, attractive green <COMMODITY>phosphate</COMMODITY> mineral; <COMMODITY>opal</COMMODITY>, a brittle gemstone; and ‘<COMMODITY>zebra rock</COMMODITY>’, a siltstone with an attractive banded texture",
" Stabilizing treatments can be applied to softer materials to prolong the quality of finish",
" In the past, an informal classification was developed to divide gems into precious and semiprecious groups largely based on their relative rarity",
" Historically, <COMMODITY>diamond</COMMODITY>, <MINERAL>ruby</MINERAL>, <MINERAL>sapphire</MINERAL>, <LOCATION><COMMODITY><MINERAL>emerald</MINERAL></COMMODITY></LOCATION>, precious <COMMODITY>opal</COMMODITY>, and <MINERAL>alexandrite</MINERAL> have been considered precious gems, with <PROJECT>all</PROJECT> others called semiprecious varieties",
" This grouping is no longer justified as each gemstone is assessed and appreciated on its individual merits, with its relative rarity determining its value",
" Gemmological terminology Most of the following chapters on <COMMODITY>gemstones</COMMODITY> and other ornamental materials contain highlighted boxes summarizing the gemmological properties of these materials",
" The boxes vary with each type of gemstone and ornamental stone and show the important identifying parameters and characteristic features of each",
" Detailed references to these terms used can be found in many standard gemmological and geological books",
" Brief explanations of some of these terms are given below",
" Appearance describes common colours, recognizable patterns, and textures characteristic of the material",
" Birefringence, also called double refraction, is a measure of the difference between the lowest and highest refractive indices of an optically anisotropic gem mineral (a mineral with a physical property that varies with crystallographic direction)",
" Minerals with high birefringence, such as <COMMODITY>zircon</COMMODITY>, may display images of their internal features that appear doubled in specific orientations",
" Double refraction is only possible in minerals that belong to non-isometric <COMMODITY>crystal</COMMODITY> systems and only when viewed in specific crystallographic directions",
" Minerals of the isometric (cubic) system are singly refractive and show no birefringence",
" Cleavage and fracture are terms used to describe splitting or breakage of both minerals and rocks caused by planes or zones of weakness along which breakages may preferentially occur",
" Fracture is an uneven breakage in any direction other than that produced by regular cleavage planes",
" A common example in rocks is conchoidal fracture, which results in concave depressions similar to bottle glass fracture",
" Fracture is seldom used as a diagnostic property of a mineral",
" Mineral cleavage describes where a mineral breaks along one or more well-defined, crystallographic planes",
" These planar surfaces are described according to the nature of the surface, such as perfect, good, fair, or poor",
" Mineral cleavage is a physical property of a mineral relating to its atomic bonding that reflects its <COMMODITY>crystal</COMMODITY> structure",
" For example, <COMMODITY>diamond</COMMODITY> has perfect cleavage along planes parallel to its octahedral symmetry",
" Cleavages defining planar surfaces in fine-grained rocks may have resulted from a number of effects including the formation of thin sheets parallel to folding pressures in <ROCK><COMMODITY>slate</COMMODITY></ROCK> and <ROCK>phyllite</ROCK> where the planar surfaces do not relate to former bedding planes",
" Alternatively, rock cleavage may result from a tendency to break along parallel layers of mineral concentrations, such as <COMMODITY><MINERAL>mica</MINERAL></COMMODITY> and <COMMODITY>quartz</COMMODITY>, that may be aligned to bedding planes or other preferred directions",
" Colour is an important guide in the identification of gem minerals and is a specific response of the eye to visible light (a range from 380 to 780 nm)",
" Colour is perceived by the eye as a combination of both reflection and transmission of light as it interacts with a gem",
" The causes of colour in gems are complex",
" Some gems contain trace amounts of elements that generate colour change in an otherwise colourless gem, such as <COMMODITY>chromium</COMMODITY> and <COMMODITY>vanadium</COMMODITY>, that may result in green <COMMODITY>beryl</COMMODITY> known as <LOCATION><COMMODITY><MINERAL>emerald</MINERAL></COMMODITY></LOCATION>",
" The presence of trace elements affects the absorption of particular wavelengths in <LOCATION>the gem</LOCATION> and results in perceived colour differences",
" The majority of gem minerals, including <COMMODITY>quartz</COMMODITY>, <COMMODITY>diamond</COMMODITY>, <COMMODITY>beryl</COMMODITY>, <COMMODITY>tourmaline</COMMODITY>, <COMMODITY>topaz</COMMODITY>, and <COMMODITY>corundum</COMMODITY>, would be colourless (allochromatic) if they contained only the elements of their ideal chemical composition",
" Other causes of colour include structural defects of the <COMMODITY>crystal</COMMODITY> lattice",
" Colour effects such as iridescence, where spectral colours are seen, result from diffraction and light interference effects caused by the physical structure of the material",
" Play-of-colour in precious <COMMODITY>opal</COMMODITY> and labradorescence displayed by some plagioclase feldspars results from the physical structures of these materials",
" <COMMODITY>Crystal</COMMODITY> system refers to the symmetry group into which a mineral is classified based on its <COMMODITY>crystal</COMMODITY> structure",
" Each mineral can be classed into one of seven (previously six) <COMMODITY>crystal</COMMODITY> systems: isometric (cubic), tetragonal, orthorhombic, hexagonal, trigonal, monoclinic, and triclinic",
" Almost <PROJECT>all</PROJECT> minerals are crystalline",
" Non-crystalline minerals including glass and resins have no characteristic external form and are termed amorphous",
" An understanding of crystallography is important in gem identification and cutting as <COMMODITY>crystal</COMMODITY> structure influences many mineral properties",
" Habit is the characteristic <COMMODITY>crystal</COMMODITY> form of a mineral, and may vary depending on the conditions of formation of the mineral",
" Habit is the easily observed expression of dominant <COMMODITY>crystal</COMMODITY> faces and is qualified by the appropriate form such as prismatic habit where describing columnarshaped crystals of, for example, <COMMODITY>beryl</COMMODITY> or <COMMODITY>tourmaline</COMMODITY>",
" Hardness is stated as a number or range of numbers according to Mohs <PROJECT>scale</PROJECT>",
" The numbers denote an order of relative hardness or resistance to scratching and have no quantitative significance",
" Thus, a mineral’s hardness figure shows whether it can be scratched by a mineral harder than itself (i",
"e",
" it is of lower hardness), or whether it scratches a mineral of lower hardness on the <PROJECT>scale</PROJECT> (i",
"e",
" it is of greater hardness)",
" The <PROJECT>scale</PROJECT>, first specified by the <LOCATION>German</LOCATION> mineralogist Friedrich Mohs, comprises 10 minerals in order of their comparative hardness from softest (1) to hardest (10): Mohs hardness <PROJECT>scale</PROJECT> 1",
" <COMMODITY>talc</COMMODITY> 2",
" <COMMODITY>gypsum</COMMODITY> 3",
" <COMMODITY>calcite</COMMODITY> 4",
" <COMMODITY>fluorite</COMMODITY> 5",
" apatite 6",
" orthoclase <COMMODITY>feldspar</COMMODITY> 7",
" <COMMODITY>quartz</COMMODITY> 8",
" <COMMODITY>topaz</COMMODITY> 9",
" <COMMODITY>corundum</COMMODITY> 10",
" <COMMODITY>diamond</COMMODITY>",
" Hardness test results establish some important distinctions between minerals of similar appearance such as <COMMODITY>beryl</COMMODITY> (hardness 7) and apatite (hardness 5)",
" Hardness is a physical property that can be easily tested in the field on rough materials",
" Mineralogists also substitute other materials in the same way using a <COMMODITY>copper</COMMODITY> coin (hardness 3) or steel point (hardness 5",
"5) Lustre is the brilliancy effect produced by light reflected from the surface of a gem",
" It is observed from a natural surface of a rock, mineral, or polished surface and its appearance is determined by the amount of incident light reflected from its surface",
" <COMMODITY>Gold</COMMODITY> and <COMMODITY><MINERAL>iron</MINERAL></COMMODITY> <COMMODITY>pyrite</COMMODITY> are both gem materials described as possessing a metallic lustre and many other gem minerals are described as having a vitreous (glass-like) lustre",
" Lustre is dependent on a combination of factors including the refractive index (RI) of <LOCATION>the gem</LOCATION> mineral and perfection of polish, which itself depends on the hardness of the material",
" Typically, minerals that are hard can be finished to a high polish and will retain the polish for longer because they are more resistant to abrasion",
" <COMMODITY>Diamond</COMMODITY> has the greatest hardness of any natural mineral, high RI, and an adamantine lustre",
" This lustre type is also displayed by other gems with high RI such as <COMMODITY>zircon</COMMODITY> and <MINERAL>demantoid</MINERAL> <COMMODITY>garnet</COMMODITY>",
" Several rock types containing minerals of low hardness, such as serpentinites, have a waxy or resinous lustre",
" Pleochroism is a general term used to describe change of colour shown by some coloured gems when viewed in different orientations",
" Pleochroism is most easily detected with the aid of a dichroscope, which allows two colours to be viewed side by side",
" Dichroic minerals show two different colours (a hue change) or different intensities of colour when viewed by a dichroscope in directions other than parallel to an optical axis",
" Trichroic minerals can display a maximum of three colours or three changes of colour",
" Dichroic minerals belong to the tetragonal, hexagonal, and trigonal <COMMODITY>crystal</COMMODITY> systems, and trichroic minerals to the orthorhombic, monoclinic, and triclinic systems",
" Minerals of the isometric (cubic) system cannot display pleochroism",
" <LOCATION><COMMODITY><MINERAL>Emerald</MINERAL></COMMODITY></LOCATION> and coloured <COMMODITY>tourmaline</COMMODITY> are examples of gem minerals that show strong dichroism",
" Cordierite (<MINERAL>iolite</MINERAL>) is an example of a trichroic gem showing three changes of colour from blue to violet-blue and yellow",
" Pleochroism (both dichroism and trichroism) can only be demonstrated in coloured gems",
" Refractive index (RI) is shown as a number or range of numbers that express the optical density or refracting power of a mineral",
" For most minerals, refractive indices range from 1",
"35 to 2",
"42",
" RI is a ratio representing the velocity of light incident on a mineral (the velocity of light in air in practice is taken as a standard = 1",
"00) and its speed when transmitted through a mineral",
" RI is measured using a refractometer and standard models can detect a range of RI values between 1",
"4 and 1",
"8",
" In gem testing, an understanding of the behaviour of RI readings from a refractometer may be used to devise the optical character of a mineral",
" RI is probably the most important information used in gem mineral identification",
" Specific gravity is the relative density of a material calculated as the mass or weight of a substance divided by the weight of an equal volume of pure water at 4°C",
" Specific gravity is a ratio without units",
" It can be determined by different methods; gemmologists commonly use a specialized balance with dual pans that permit weighing of the material in air and fully immersed in water",
" Calculation of specific gravity is derived using the formula: (weight of the material in air)/(weight of equivalent volume of water)",
" Transparency or diaphaneity expresses the degree to which light can be transmitted through a material",
" Terms include grades of transparency ranging from fully transparent, where objects can be viewed through the mineral, to partially transparent as in semitransparent to translucent, where some light is transmitted",
" No light is transmitted through opaque materials such as <COMMODITY>gold</COMMODITY>",
" Most minerals and rocks have some degree of translucency in thin section",
" Ultraviolet (UV) light and fluorescence",
" Ultraviolet light responses to <COMMODITY>gemstones</COMMODITY> are uncommon although photoluminescence reactions shown by some gem minerals may provide a useful indication of a gem’s identity",
" Fluorescence is the emission of visible light by certain minerals when exposed to UV light",
" Ultraviolet light, also known as black light (because it is invisible), is commonly categorized as short or long wave, based on the wavelength range produced by filters or mercury discharge lamps",
" Long-wave UV has a range from 315 to 400 nm, and short-wave UV from 200 to 280 nm",
" Prospectors’ UV lamps are likely to have filters specifically made for detecting UV responses of certain economic minerals such as scheelite (calcium tungstate) and may emit a mix of long- and short-wave UV",
" Gem minerals that commonly respond to UV light include diamonds, <COMMODITY>topaz</COMMODITY>, and some synthetic gem materials such as cubic zirconia and glass (paste)",
" Ultraviolet light will commonly cause surface crusts of <MINERAL>opaline</MINERAL> <COMMODITY><MINERAL>silica</MINERAL></COMMODITY> to fluoresce white or green, and <COMMODITY>calcite</COMMODITY> pink",
" Gemmology books provide reference tables on UV responses and information on personal safety for using UV light"
]
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"sentence": " an understanding of crystallography is important in gem identification and cutting as <commodity>crystal</commodity> structure influences many mineral properties"
},
{
"sentenceNumber": 40,
"rocks": null,
"commodities": [
"crystal"
],
"minerals": null,
"sentence": " habit is the characteristic <commodity>crystal</commodity> form of a mineral, and may vary depending on the conditions of formation of the mineral"
},
{
"sentenceNumber": 41,
"rocks": null,
"commodities": [
"crystal",
"beryl",
"tourmaline"
],
"minerals": null,
"sentence": " habit is the easily observed expression of dominant <commodity>crystal</commodity> faces and is qualified by the appropriate form such as prismatic habit where describing columnarshaped crystals of, for example, <commodity>beryl</commodity> or <commodity>tourmaline</commodity>"
},
{
"sentenceNumber": 50,
"rocks": null,
"commodities": [
"talc"
],
"minerals": null,
"sentence": " <commodity>talc</commodity> 2"
},
{
"sentenceNumber": 51,
"rocks": null,
"commodities": [
"gypsum"
],
"minerals": null,
"sentence": " <commodity>gypsum</commodity> 3"
},
{
"sentenceNumber": 52,
"rocks": null,
"commodities": [
"calcite"
],
"minerals": null,
"sentence": " <commodity>calcite</commodity> 4"
},
{
"sentenceNumber": 53,
"rocks": null,
"commodities": [
"fluorite"
],
"minerals": null,
"sentence": " <commodity>fluorite</commodity> 5"
},
{
"sentenceNumber": 55,
"rocks": null,
"commodities": [
"feldspar"
],
"minerals": null,
"sentence": " orthoclase <commodity>feldspar</commodity> 7"
},
{
"sentenceNumber": 56,
"rocks": null,
"commodities": [
"quartz"
],
"minerals": null,
"sentence": " <commodity>quartz</commodity> 8"
},
{
"sentenceNumber": 57,
"rocks": null,
"commodities": [
"topaz"
],
"minerals": null,
"sentence": " <commodity>topaz</commodity> 9"
},
{
"sentenceNumber": 58,
"rocks": null,
"commodities": [
"corundum"
],
"minerals": null,
"sentence": " <commodity>corundum</commodity> 10"
},
{
"sentenceNumber": 59,
"rocks": null,
"commodities": [
"diamond"
],
"minerals": null,
"sentence": " <commodity>diamond</commodity>"
},
{
"sentenceNumber": 60,
"rocks": null,
"commodities": [
"beryl"
],
"minerals": null,
"sentence": " hardness test results establish some important distinctions between minerals of similar appearance such as <commodity>beryl</commodity> (hardness 7) and apatite (hardness 5)"
},
{
"sentenceNumber": 62,
"rocks": null,
"commodities": [
"copper"
],
"minerals": null,
"sentence": " mineralogists also substitute other materials in the same way using a <commodity>copper</commodity> coin (hardness 3) or steel point (hardness 5"
},
{
"sentenceNumber": 65,
"rocks": null,
"commodities": [
"gold",
"iron",
"pyrite"
],
"minerals": [
"iron"
],
"sentence": " <commodity>gold</commodity> and <commodity><mineral>iron</mineral></commodity> <commodity>pyrite</commodity> are both gem materials described as possessing a metallic lustre and many other gem minerals are described as having a vitreous (glass-like) lustre"
},
{
"sentenceNumber": 68,
"rocks": null,
"commodities": [
"diamond"
],
"minerals": null,
"sentence": " <commodity>diamond</commodity> has the greatest hardness of any natural mineral, high ri, and an adamantine lustre"
},
{
"sentenceNumber": 69,
"rocks": null,
"commodities": [
"zircon",
"garnet"
],
"minerals": [
"demantoid"
],
"sentence": " this lustre type is also displayed by other gems with high ri such as <commodity>zircon</commodity> and <mineral>demantoid</mineral> <commodity>garnet</commodity>"
},
{
"sentenceNumber": 75,
"rocks": null,
"commodities": [
"crystal"
],
"minerals": null,
"sentence": " dichroic minerals belong to the tetragonal, hexagonal, and trigonal <commodity>crystal</commodity> systems, and trichroic minerals to the orthorhombic, monoclinic, and triclinic systems"
},
{
"sentenceNumber": 77,
"rocks": null,
"commodities": [
"emerald",
"tourmaline"
],
"minerals": [
"emerald"
],
"sentence": " <location><commodity><mineral>emerald</mineral></commodity></location> and coloured <commodity>tourmaline</commodity> are examples of gem minerals that show strong dichroism"
},
{
"sentenceNumber": 78,
"rocks": null,
"commodities": null,
"minerals": [
"iolite"
],
"sentence": " cordierite (<mineral>iolite</mineral>) is an example of a trichroic gem showing three changes of colour from blue to violet-blue and yellow"
},
{
"sentenceNumber": 97,
"rocks": null,
"commodities": [
"gold"
],
"minerals": null,
"sentence": " no light is transmitted through opaque materials such as <commodity>gold</commodity>"
},
{
"sentenceNumber": 100,
"rocks": null,
"commodities": [
"gemstones"
],
"minerals": null,
"sentence": " ultraviolet light responses to <commodity>gemstones</commodity> are uncommon although photoluminescence reactions shown by some gem minerals may provide a useful indication of a gem’s identity"
},
{
"sentenceNumber": 105,
"rocks": null,
"commodities": [
"topaz"
],
"minerals": null,
"sentence": " gem minerals that commonly respond to uv light include diamonds, <commodity>topaz</commodity>, and some synthetic gem materials such as cubic zirconia and glass (paste)"
},
{
"sentenceNumber": 106,
"rocks": null,
"commodities": [
"silica",
"calcite"
],
"minerals": [
"opaline",
"silica"
],
"sentence": " ultraviolet light will commonly cause surface crusts of <mineral>opaline</mineral> <commodity><mineral>silica</mineral></commodity> to fluoresce white or green, and <commodity>calcite</commodity> pink"
}
]