Obsidian Jewelry

Where the name comes from: The Ancient Greek philosopher Pliny mentions "Obsidianus" a volcanic glass named after Obsius who found a similar stone in Ethiopia.
Where the stones come from: World wide in areas of a particular type of volcanic eruption - rhyolitic, where the lava has cooled extremely quickly.
Chemistry:  A volcanic glass. 70-75% Silicon dioxide, SiO₂ - the remainder is magnesium and iron oxides, MgO and Fe₃O₄
Colors: Black, dark brown, dark green, also with a range of sheens from yellow, blue and rainbow caused by tiny included air bubbles. Snowflake obsidian has inclusions of white crystals in a black matrix.
...and another thing: As a form of glass, obsidian can form very sharp cutting edges, even today it can be used for scalpel blades which are much sharper and smoother than steel.
 

Prices correct at time of writing - subject to change