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Properties
Obsidian is conchoidal, which means it forms sharp flakes whenever it is cracked. It has a hardness of 5 to 5 1/2 on Moh's scale of mineral hardness. It has no facets as a crystal does and occurs in the field either as a layer in rocks or as a nodule. Obsidian is a translucent mineral, meaning some light can pass through it, although not as much as through minerals which are completely clear.
Color
Black is the most common color of obsidian rock. Obsidian can also occur as a green or brown and more rarely as orange, yellow, blue or red, or as a combination of these colors in the form of "rainbow" obsidian. The inclusion of trace elements are thought to be the cause of the rarer colors of obsidian. Obsidian can also contain tiny shards of crystals that cause it to be iridescent; this type is called either golden obsidian or silver obsidian.
Obsidian in Tools
Obsidian was used as a cutting tool by early humans due to its sharp edges. Large flakes of obsidian were broken from a large nodule and transported back to camps where they could be further worked into knife blades, hatchet blades and spear heads. Obsidian is still used for this purpose today by surgeons. Obsidian scalpel blades are thinner and sharper than surgical steel, which makes them ideal to use in precise surgical procedures.
Obsidian in Jewelry
Due to its luster, obsidian has been made into jewelry for thousands of years. Due to its softness, artisans can carve it or bore holes into it to make beads. Obsidian continues to be used in this way today. Obsidian is also used in the gem-crafting industry to make opals. In this practice, a thin slice of obsidian is used as a backing for lab-crafted opals. The dark color of the obsidian causes the iridescence of the opal to be viewable and the opal to be less brittle.