The Romans named this striking gem Opalus, a Latin term synonymous with precious stones. The opal was considered a symbol of love and hope in ancient Rome. In AD 75, Pliny the Elder, the Roman scholar, observed how this remarkable gemstone incorporated the rich colours associated with other rare gemstones such as sapphire, ruby and emerald. The rare black opals which exhibit a beautiful array of colours compete with the attraction of these traditional precious gemstones.
The majority of opal deposits are thought to have been formed up to 30 million years ago, although the opals found in Mintabie, one of Australia’s main opal sources, could be up to 40 million years old. Australia is the centre of world opal production and the Lightning Ridge mine is the main source of the black opal, which was discovered there in 1902.
Opals are formed when the dry ground becomes saturated by the seasonal heavy rainfall. The rain carries the dissolved silica down into the rock cavities. The heat of the sun then then evaporates the water off when the dry season returns, leaving behind the opals formed from the silica deposits.
The opal’s elegance comes from its three characteristics:
Colour – the background colours and the play-of-colour
Pattern – the arrangement of the play of colour
Clarity – the transparency and number of inclusions
The appearance and quality of one opal can vary greatly to that of another; an individual opal can even change in appearance with a shift-of-play in colour and pattern. The weather, time of day, viewing angle and lighting all contribute to an opal’s colour and pattern.
A black opal can only be classed as a truly black opal if its background colour appears black in reflected light. The background colours of black opals do vary between deep, dark shades of black, grey, blue, green or brown. However, the darker the background colour, the more valuable the stone becomes, and the more desirable.