A: Obsidian is a natural volcanic glass. Its properties can vary with the exact composition, but the numbers normally cited for it go like this: Refractive index of 1.50 (+-.02), Specific gravity of 2.40 (-.07, +.10), and hardness of 5 to 5.5. Most is black and nearly opaque, though other colors and greater transparency are also found. Popular are varieties with white “snowflake” patterns formed by partial devitrification, or with brown/black patterns (called mahogany obsidian), or with various types of inclusions giving it a sheen, reflectance, color or patterns of various types.

Basically, the stuff should look and feel like black glass. It is commonly imitated or confused with molded glass products (mold marks identify the fake), although glass in cut and polished form, especially some types of glassy industrial slags, can be quite hard to distinguish. I’ve often found the latter types in Mexican material represented by locals as obsidian, which can be tricky to identify. Usually it’s denser black color, no patterns, and too many gas bubbles which are the clues.

It is often imitated by black plastics of various types as well. These can also show mold marks, and are easily separated by the much softer nature of the plastic (a razor blade will “bite”), as well as a “warm” feel, lighter specific gravity, and all the other ways in which plastic is easily distinguished from glass…