Below is a brief description of the most popular coloured gemstones requested by customers.
The name Sapphire (Greek – blue) used to be applied to various stones. In antiquity and as late as the Middle-Ages, the name sapphire was understood to mean what is today described as Lapis Lazuli. Around 1800, it was recognised that sapphire and ruby are gem varieties of corundum. Corundum is the hardest mineral after diamond which is highly beneficial for durability when being worn as jewellery. Today, corundums of all colours (except red) are called sapphires. Red varieties are called rubies. Sapphires are available in a range of colours such as blue, pink, orange, yellow, green, purple and black.
Ruby is named because of its red colour (Latin - Rubeus). It was not until around the 1800s that rubies and sapphire were recognised as belonging to the corundum group. Before that date, gemstones such as red spinel and garnet were also designated as ruby. The red colour varies with individual deposits so it is not possible to determine the source area from the colour, as each deposit yields different tones. The designation “Burma ruby” or “Siam-ruby” is erroneous and refers more to the quality than origin. The most desirable colour is “pigeon’s blood”, pure red with a hint of blue. As a rough stone, ruby appears dull and greasy but, when cut, the lustre can approach that of a diamond.
The name is derived from the Indian word for “stone”. It is divided into three groups: The opalescent precious opals, the yellow-red fire opals and the common opals. Their physical appearances vary considerably.
The special characteristic of these gems is their opalescence, a rainbow- like iridescence which changes with the angle of observation. Until the 1960’s, this was thought to be caused by the refraction of light from the thin surface layers. The real cause was discovered under the electron microscope using a magnification of 20,000. Tiny spheres (0.001mm in diameter) of the mineral cristobalite layered in siliceous jelly cause the reflection or interference appearances. Strictly speaking, precious opal is not really amorphous.
Opal always contains water; the content does vary but it can contain as much as 30%. Over time this can the stone can lose water, cracks and opalescence diminishes. This can temporarily be restored by saturation with oil or water.
Care must be taken during setting as opal is sensitive to pressure and knocks as well as being affected by acids and alkalis. A little heat can also evaporate the water contained within the opal.
Up until the turn of the century, the andesite lavas in the Czech Republic supplied the best qualities. Then the Australian deposits were discovered. Famous deposits in New South Wales are at Lightning Ridge and White Cliffs. South Australia at Coober Pedy and Andamooka; in Queensland at Bulla Creek and Burcoo River. Further deposits are found in Brazil, Guatemala, Honduras, Japan and the US .(Nevada) In Europe precious opal has been thought of as unlucky, but in the Orient it stands for loyalty and hope. The play of colour is seen best in a cabochon cut stone. Very thin pieces of opal are sometimes mounted on a piece of common opal or onyx: this is the opal doublet. Triplets are also made with a protective layer of rock crystal. Colour- white-green-blue- orange.
The name emerald derives from the Greek Smaragdos, which in turn came perhaps from the Persian. It means “green stone” and, in ancient times, referred not only to emeralds, but also probably to most green stones. Emerald together with aquamarine and beryl, belongs to the beryl group, being the most precious of the group. It’s is incomparable, and is therefore called “emerald green” (not only in mineralology). Only the finest qualities are transparent. Often the emerald is clouded by inclusions (liquid or gas bubbles, healing cracks and foreign crystals). These are not necessarily classified as faults, but are evidence as to the genuineness of the stone as compared with synthetic and other imitations. The most desired colour is a deep green which is more valuable, even with inclusions, than a pale and clean quality. All emeralds are brittle and combined with internal stresses, sensitive to pressure.
Aquamarine, together with emerald and beryl, belong to the beryl group; it is so named (Latin-Water of the sea) because of its sea water colour. Lower qualities are heated to 752°C/400°C to change them to the desired aquamarine blue. It is brittle and sensitive to pressure. It is more frequently transparent than emerald. There are aquamarine deposits in all continents. The most important ones are in Brazil (Minas Gerais, Bahia, Esperito Santo). All other deposits are only of local importance: Australia (New South Wales), Burma,, Sri Lanka, India. Kenya, South Africa, Tanzania and the U.S. Colour: Light blue, Blue, Blue – green.