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 often known as Sunstone. Sunstone, moonstone and aventurine are all gem quality forms of oligoclase.
Olivine Olivine, also known as chrysolite, is a very common mineral, but specimens large enough to be cut into gemstones are quite rare. In its gemstone form it is more often known as peridot; a bright yellow-green translucent crystal, and the birthstone for those born in August. Olivine forms in magma where it crystallises quickly as the magma cools, and it has also been found in meteorites. The meteorites containing olivine are thought to have come from an extinct planet that once orbited our sun between Mars and Jupiter – you learn something new every day!
Olivine is a very interesting mineral. You may like to read more about it here.
Omphacite Omphacite is closely related to jade, being a green mineral with a high jadeite content. For that reason it is more often known as Omphacite Jade. Omphacite Jade has a silky opaque appearance, but some almost translucent specimens have been found. Because its relatively soft, it's often polished en cabochon or carved intaglio or cameo. It makes a very pretty gemstone or ornamental mineral.
Onyx, like agate, is a variety of chalcedony, but where agate generally has bands in rings, the white bands in onyx are more linear. Many people think of onyx as being black, and while it does naturally occur in black, most black onyx has in fact been dyed. Natural onyx occurs in black, browns and reds. Onyx has been prized by people for thousands of years. Romans would carve intaglios into onyx for their wax seals, and it's was a popular choice for rosary beads too. When onyx was scarce, it was often 'created' by dying agate; a practice that still exists today to create black onyx. Because the bands are in layers, onyx creates beautiful cameos and intaglios where the top layer is carved through to reveal the image in the white second layer.
Opals are available in various 'types' and colours, but did you know that opals are formed from rain? When rain makes its way into natural rock crevices it reacts with the minerals present to form a silica, then when the rain evaporates the silica hardens into opal. Because of this, all opals are comprised of up to 30% water. Opals have even been found on Mars, proving that water once flowed there, and in meteorites. In most cultures opals are said to bring good luck, but in others (typically western), they are thought to be unlucky. Although there are many different types of opal available, all can be placed into one of two categories; precious opal, and common opal (otherwise known as potch). Precious opal displays the colourful characteristics and light play of the opals we love in jewellery, while common opal is one flat colour and used only for building up layers beneath precious opal to create a larger gemstone from small precious opal pieces. Black opal, which isn't black at all, but rather an opal with darker base to better display the colours, is the rarest and most precious. Opals are found throughout the world, but the best opals are found right here in Australia.
You can find lots of interesting information about opals here.
There are multiple forms of Opalite to be aware of, the most commonly referenced being: 1) Opalite is a trade name for manufactured synthetic opal, created from glass and usually milky white in colour. When placed on a white background the opalite will have a pink shimmer; when placed on a dark background the shimmer is blue. Other backgrounds and lighting can change the shimmer to different colours. Some less scrupulous sellers will try to sell opalite as moonstone.
2) Opalite is also the name given to a naturally occurring stone formed from volcanic ash. It's occurs mainly in green, but also in lavender and purple. Natural opalite will often present with a 'chatoyency' - a cat's eye effect.
Orpiment Orpiment is a very dangerous crystal, containing about 60% arsenic! It's rare in large crystal form, but when found and polished en cabochon it is very beautiful. Orpiment is an intense amber gold colour that was a popular pigment when ground down for artists. It's also been used in ancient medicines before it's toxicity was realised. Orpiment is a soft mineral that will decompose over time with exposure to air, forming a white powder coating that is highly toxic. Therefore, if you do happen to have any orpiment in your collection, it must always be carefully stored and never handled with bare skin.
Orthoclase is rare and pretty gemstone of the feldspar family (also containing moonstone, sunstone, and oligoclase). It forms in translucent crystals of champagne yellow to greenish yellow, and looks beautiful when faceted and set into jewellery. The largest example of an orthoclase gem weighs 250carats and is currently on display at the Smithsonian National Museum of Natural History in Washington D.C.
We'll continue our look at gemstones when we bring you gemstones beginning with P in a few weeks time.
Image Attributions (where required): Oldhamite: By Leon Hupperichs, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=35072128 Olivine: By Rob Lavinsky, iRocks.com – CC-BY-SA-3.0, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10448817
Onyx: By Gryffindor [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC BY 2.5 (https://creativecommons.org/licenses/by/2.5)], from Wikimedia Commons
Opal: By Ra'ike (see also: de:Benutzer:Ra'ike) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)], from Wikimedia Commons