Gemology: O is for ...
Welcome to the 15th instalment of our gemology guide: Gemstones beginning with O. Today we bring you Obsidian, Oldhamite, Oligoclase, Olivine, Omphacite, Onyx, Opal, Opalite, Orpiment, and Othoclase. How many of these have you heard of before? Obsidian Obsidian is a natural black, variegated black, red, or orange volcanic glass formed from cooling lava. It's been used by people for many centuries in the making of arrow heads (it cleaves very sharply!), and for tools and decoration. Even today, the thin sharp edges that can be produced in obsidian make it useful in the medical industry, with scalpels being made with obsidian blades for very fine surgery. In jewellery obsidian is prone to scratching so it is usually polished en cabochon, though faceted gemstones can be very striking.
You can learn more about interesting facts about obsidian here.
Oldhamite is a meteoric mineral first identified in the Bustee Meteorite found in India in 1862, and found in other meteoric sites around the world. It's not particularly attractive, being lumpy and coal like in appearance, in shades of greyish-brown. Oldhamite is not really suitable for jewellery use unless you're of a particularly quirky nature.
Oligoclase Oligoclase is a feldspar mineral like albite and moonstone. It generally occurs in a colourless or white form, though it can be tinged by colour depending on the inclusions. Oligoclase with hematite inclusions is more