34th Wedding Anniversary gift
Opal is a non-crystalline gem material. Most gemstones are formed in one of the seven crystal systems but opal is what is known as an ‘amorphous’ material, meaning ‘without form’. Opals are created from microscopic spheres of silica gel and it is the arrangement of these spheres that produces the beautiful play of colour that we all love. Opal is a relatively soft material, about the same as glass with a score on Moh’s scale of Hardness of 5.5 – 6.5. As such it is highly recommended to remove opal jewellery when doing any type of hard manual work to avoid damaging it.
The effect of the play of colour in opal is similar to the iridescence seen in a layer of oil on water when caught in sunlight. When the silica spheres are neatly arranged, the light passes from the air in to the opal and bends (refracts). Some of this light bounces off the spheres and reflects back to the eye showing colour. This process continues down through the layers of spheres, refracting and reflecting. The different colours seen in the opal are due to the different sizes of silica spheres. Smaller spheres will produce colours nearer the blue end of the visible light spectrum whereas larger spheres will show colours from the red end of the spectrum.
The main source for gem quality opal has always been Australia. Certain parts of the continent are renowned for particular types:
Lightning Ridge in New South Wales – Black opal
Yowah, Queensland – Yowah ‘nut’ opals
Andamooka, South Australia – Crystal seam opal, opalised fossils, black opal, matrix opal. The Andamooka Opal was presented to Queen Elizabeth II during her post coronation tour in 1953/54, it is also known as the Queen’s Opal. The cut stone weighs in at 203 carats, just over 40grams. It was found in 1949 in the Andamookaopal fields.
White Cliffs, New South Wales – ‘Pineapple’ opals, so named due to their resemblance to the spiky top of a pineapple.
Coober Pedy, South Australia – major source of opal. The ‘Olympic Australis’ opal was found at “8 Mile Opal Field” in Coober Pedy in 1956. It weighs 3.4KG/7.5lbs. It weighs 17,000 carats.
In 1994, the first ‘Welo’ opals were discovered in Ethiopia. They originally came from the Menzgishe district in the northern part of Shewa Province in Ethiopia. Today up to 10% of opals are mined in Ethiopia.
In 2008, more opals were found in the Tsehay Mewcha region of Welo Province and these Ethiopian opals were the first ones suitable to be cut.
Welo opals are found in cliffs, approximately 10,000 feet above sea level. The area is not accessible by vehicles or animals and the miners have to descend 1000 feet via very narrow, steep footpaths and they are reliant only on the equipment they are able to carry, including very simple hand tools. Sadly there have been a number of fatalities but is has improved after intervention and training sessions and basic safety equipment was provided by the Ministry of Mines and Energy.
Australian opals tend to be kept protected from the sun after mining to prevent them from drying out too quickly and cracking. They are also traditionally cut wet as this prevents the cutter from inhaling as much silica dust. Cutting the gems whilst wet also limits the risk of the stone overheating and cracking.
Ethiopian opals allow water to be absorbed into them which makes them transparent and they lose their play of colour. For this reason, the miners of these gems allow the stones to dry out before offering them to dealers. These gems are also softer than the Australian opals and this means that some stone cutters will cut a dry Ethiopian opal as it is easier to see the play of colour. In this instance the cutter wears a mask and is in a well ventilated area to avoid the risks of silicosis which causes permanent lung damage by breathing in the silica particles as the opals are cut.
Mexican Fire Opals. A vibrant orange colour, rarely with a play of colour. From the 1960’s – 1970’s many opal mines were in existence in the Querétaro state of Mexico. The opals from here were mainly found in the mountainous areas. However, the miners in the regions today say it is much harder to find gem quality opals with fire and play of colour and those that are found can command very high prices.
BEWARE OF COMPOSITE STONES: You may see opals advertised as ‘doublets’ or ‘triplets’ These are not solid opal. They are either a thin slice of opal with a dark backing added to it to make it look more like a more expensive ‘black opal’ – two layers, hence doublet. The other, a triplet, is where a dark backing is applied to the thin slice of opal and then a clear top layer of quartz or glass is added as well. It it wise to be wary of any opal that is set in a closed back setting i.e. where you cannot view the back of the stone, as you cannot be certain that you are seeing a solid opal.
Cleaning. DO NOT allow anyone to place your opal jewellery in an ultrasonic cleaner as the vibrations can cause cracking to the stone.
Below is a video showing some composite opals and some natural solid Australian opals along with some Mexican fire opals. We hope you enjoy watching it. Please see our channel for more short gemstone videos.
