Scrap is the least common denominator of useful value on Calydon. It can be used the synthesize most other goods via the Cavelink from a player's Matrix. "Scrap" refers to mixed raw material that is a combination between the three flavors of scrap: inorganic, metal, and organic.
Most items require some of each of these components to synthesize. When scrap is consumed for crafting, the change is left over in one of the three subcomponents of scrap.
One of each subcomponent of scrap will automatically collapse back into one unit of scrap when they are in a character's inventory. (Think like gold-piles in Diablo I inventory.)
Inorganic scrap:
Minerals
Rocks
Metal - Scrap metal isn't worth much, but it is useful and often comes in handy to repair or build tools. Metal can be strong, malleable, and can conduct electricity. Metal is a common currency on Calydon.
Aluminum - A fairly weak, but highly reflective metal. Can be used in the manufacture of many things, and fail is very useful for storing perishables.
Cog
Component
Gear
Iron - A rudimentary metal.
Ore - Unrefined metal.
Plate
Screw
Siderophore
Spring
Steel - Great for building fences and weapons.
Organic scrap:
An intact ribcage. Thankfully the number of ribs suggest they're not human. At least you don't think it does; you never took anatomy.