Natural obsidian test stones give precise readings when testing for gold purity. Obsidian is a volcanic glass produced when lava becomes cold. It is been used for years to make the sharpest knives. Obsidian test stones, on the contrary to rougher test stones, have a perfectly smooth surface that provides more consistent and reliable readings. Transline#: gt33. Size: 2x1.5. Color: black. Sold Individually. Case Qty: 1 .