Description: Garnets have been prized as gems for over 5000 years. Most people associate the word "garnet" with a red gemstone; however, they are often surprised to learn that garnet occurs in many other colors (black, brown, orange, pink, and green). The word ‘garnet’ comes from the Latin word granatus, which means ‘seed-like.’ In contrast, industrial grade garnets are used as abrasives (cutting, water blasting, powders). Garnets are found in a wide variety of rocks - igneous, metamorphic, and sedimentary.
Chemical Formula: X•Y•Z2•O6, where
X = Ca, Na, Fe++, Mg, Zn, Mn, or Li;
Y= Mg, Fe+++, Fe++, Cr, Al, Co, Mn, Sc, Ti, or Vn;
Z = Si, Al, or a combination of both.
Crystal Form: The main crystal shapes of Garnets are dodecahedral and trapezohedral. Both shapes are polyhedrons, which are three-dimensional geometric figures with flat faces.
Crystal System: Isometric
Color: Wide variety, from green to red to pink to violet, also black, and brown and a variety of tints of all.
Streak: Colorless, white or pale shade of garnet color
Luster: Vitreous (glassy) to dull
Fracture: Irregular/Uneven, Sub-Conchoidal
Cleavage: Most pyroxene minerals exhibit two directions of cleavage that intersect at approximately 87o and 93o. Cleavage fragments often have a nearly square cross-section
Hardness: 6.5 to 7.5 on Moh’s scale
Density/Specific Gravity: 3.58 - 4.32 g/cm3
Magnetism: N/A
Taste: N/A
Hydrochloric acid: N/A
Radioactivity: N/A
Fluorescence: very weak under long wave ultraviolet
Distinguishing Physical Properties:
Crystal form: Commonly found in twelve-sided crystals (dodecahedrons)
Color: Common reddish brown color, but there are other color garnets (green), often appear as obvious small red-brown spheres in metamorphic schists
Cleavage: lack of cleavage
Photograph Attribution: Garnet specimen photograph by Sean C. Murphy, 2020.