Opal is one of the most recognisable and enjoyed gems used in jewellery today, but Opal is not seen in jewels much before the 1890’s (Austro-Hungarian jewellery being an exception), in fact foiled Opaline glass is much more common particularly in Georgian jewellery.
At their best displaying a fascinating range of colours which changes as the light touches and diffracts against the spheres within their internal structure, there are many varieties of Opal, but the most commonly seen in precious jewellery is the the “noble”, or precious Opal which most often has a milky pink/white background with good 'play' of the rainbow colours within.
Other varieties include the Fire Opal, or reddish/orange Opal, which has also occasionally a fine play of colours and is found mostly in Mexico; the oval Jasper, which contains oxide of iron, and is found in the neighbourhood of the Geysers of Iceland; the wood Opal or Opalized wood, of which huge masses are met with in Hungary, Tasmania and other parts, and newly available; the Ethiopian Welo Opal.
Put simply; Opal forms in crevices in rocks, as explained by Opals Down Under; 'Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone, and carries this silica-rich solution into cracks and voids , caused by natural faults or decomposing fossils. As the water evaporates, it leaves behind a silica deposit.'
The Opal – from “Diamonds and Precious Stones” by Harry Emanuel FRGS, published in 1867.
“This magnificent gem is composed of silica in an amorphous state, mixed with water, and is in reality the same mineral as quartz, with the addition of 6 or 7 per cent of water: it never occurs in a crystallized form, it has a vitreous lustre inclining to resinous, is numbered in the table of hardness from 5.5 to 6.5.'
Opals are always cut en cabochon on both sides, and the true beauty of the gems only display themselves when the stone is moved about, as then the 'fire' within a fine Opal really appears in a full range of rainbow colours, which dependent on the type can display in different 'patterns'. The Opal is cut on and polished with emery, tripoli and water, finishing with cerum oxide, taking great care not to heat the stone too much by friction as they are very brittle and require very delicate handling. This is something we often need to do to restore the vintage Opal jewels that we sell, in particular those worn in rings.
Historically within fine western jewellery, the precious Opals used in jewellery were sourced from the Czech Repulbic and Slovakia and first documented in the 14th century. It is found in the claystone porphyry at Czernowitza; between Kaschau and Esperies in Hungary and occasionally near Frankfurt.
Austro Hungarian jewellery shows Opals from Hungary usually of the milky pink/orange variety which lack the bright colour changes found in Australian Opals. This source of Opal ceased to be mined since the early 20th century.