Sapphire is a corundum kind, a gemstone of different shades, mainly of the dark blue colour. The chemical formula is Al2O3 (aluminium oxide).
Sapphire meets more often than ruby. The most valuable colour is dark blue; Burmese and dark blue Kashmir sapphires are the most popular gemstones. Sapphires of Kashmir were extracted on the border of India and Pakistan; gems have a deep dark blue colour which is sometimes described like cornflower blue. Dark blue Burmese sapphires have an intensive bright-dark blue colour with a lung violet shade.
Sri-Lankian dark blue sapphires became very popular. Their colour varies from middle-dark blue with a violet shade to a pastel dark blue shade. The stones are often painted non-uniformly, and sri-lankian stone-cutters take it in the attention, guaranteeing, that there is some colour in kaletta of the stone. That’s why, a stone shows a good colour, even at a sideways sight it is basically colourless. Thai dark blue sapphire is dark enough. It is made in considerable quantities in the stone-cutting workshops. The Australian sapphires at the artificial illumination seem dark, therefore the material is sent to Thailand for a cutting and heat treatment to clarify the stones and to make them more transparent. However, it leads to that sapphires get a greenish shade.
The African dark blue sapphires have various shades. Nigerian stones are often very dark, but in Tanzania high quality and colour crystals are made. Sapphires from the State of Montana are usually painted in pastel violet-dark blue or steel dark blue colour which can be rough, and inclusions are possible. Usually, the American sapphires are very small, but they are the natural stones.
Fantasy sapphires can be also of other colours. Pink sapphire became very popular; it is constantly used in jewellery. The colour can vary from gentle-pink to intensive purple-red. In an ideal, there should be a uniform intensively-pink colour, without impurities of lavender or brownish shades. Natural bright-pink sapphires have been raised in cost and are sold at a price of dark blue sapphire of an average grade. Yellow sapphires are usual. Their colour scale varies from pale yellow to intensively-amber. It is usually named east topazes. Padparadsha is the rare and very expensive sapphire which colour is pink and orange simultaneously. It is extracted in Sri Lanka. Colourless sapphire is free from any impurities. The stone of the small size is easy for finding.
Sapphires have inclusions which prompt their origin. For example, Thai sapphires often contain the tiny liquid drops reminding the prints of fingers. Rutile needles in sapphire provide not only the “silkiness”, but also asterism. In an ideal, star sapphires should have an intensive transparent dark blue colour with minimum hexagonal banding and the accurate stars located in the centre of the stone. Cracks, hollows and the rough bases are usual defects. Among Thai and Sri-lankian stone-cutters there is a practice of gluing the star sapphires bases if during a cutting any site breaks. As a result the stone looks accurately, and it relieves the installation into a frame, but through some time glue is painted in yellow colour, spoiling its view.
The sapphires, which change their colours, are an unusual phenomenon. Gems can simply show the various shades of dark blue colour, depending on the type of a light source or completely change the colour from blue-violet at day light to red or reddish-violet at the artificial illumination.
Ruby is the corundum painted from the crimson-purple, red to the yellow-red colour. The most beautiful colour is pure red colour, without a hint on dark blue shades. Burmese blood-red rubies «pigeon blood» possess this fine colour, this makes them the most demanded and expensive. As, since 1963, mines in Mogok are under the strict control of the government of Myanmar and are leased only to the citizens of this country, the stones are not impossible to buy there. The citizens transport a precious material through border to Thailand.
In Vietnam and Tanzania rubies of pure red colour are also extracted, but the majority of them at heating have a violet or brownish shade. Tanzanian rubies usually are opaque, connected with green ciozit, and also are used for decorative carving on the stone. Sri-Lankian rubies usually have a colour from light-red to the crimson. Once Thai rubies were not accepted by dealers because of their similarity with garnets. Thai rubies have a darker, crimson-red colour, but in whole, they are the “pure” stones with minimum inclusion of the rutile needles.
Inclusions help to define the country and the area of origin of the stone. For example, Burmese rubies usually contain rutile, calcite, apatite, olivine, sphen and spinel inclusions. Often there are the discal breaks round the natural mineral inclusions; it is a sign of thermal processing. Some rubies fluoresce in ultra-violet beams, and especially good stones will be fluoresce even at sunlight. When rutile needles in ruby are leveled, six-beam asterism arises. The star in ruby should be transparent and painted in red colour, instead of muddy and brownish-violet. The star beams must represent the narrow silver-white lines which reach the stone bases. Rubies can have an effect of “the cat’s eye”.
Rubies are cut in the country where they are extracted. Pleochroism dictates the orientation of the material with the table located so that a stone had the best shade of the red colour. Then ruby is cut with the step or mixed cutting that provides the maximum weight.
Estimation of corundum
Sapphires and rubies are estimated according to the colour, transparency, size, cutting and proportions. Estimating of dark blue sapphires, the frequency and intensity of dark blue colour are the most important. In an ideal, it should have the violet shade without grey or greenish tones. It is necessary to estimate a light or dark stone is, zoning of the colour and a stone transparency. Non-uniform zoning of the colour reduces the value and beauty of the stone. Inclusions unessentially are considered as a defect. They can show, whether the stone has been processed, or will help to define its origin. The big gems are expensive, but the small stones of good colour and transparency can cost also very expensively.
Sometimes colourless sapphire is used as alternative of diamond; at a good cutting, these stones can have some “game”. At the small sizes it is firmer stone than white topaz and quartz which in a frame can crack.
The specification of corundum
Hardness: 9 on Moos scale
Density: from 3,95 to 4,03
Factor of refraction: from 1,762 to 1,778
Crystal lattice: triangular
Corundum’s crystals can be in the form of a barrel-shaped 6-angles pyramid with two sharp ends or plates. Corundum is found in the volcanic or metamorphic rocks and in the alluvial deposits.
Deposits of sapphires: Tanzania, Nigeria, Madagascar, Kenya, Malawi, Australia, Brazil, Myanmar (Burma), Pakistan (Kashmir), Sri Lanka, Thailand, the USA (Montana).
Deposits of rubies: Afghanistan, Australia, Cambodia, India, Kenya, Myanmar (Burma), Pakistan (Kashmir), Russia, Tanzania, Thailand, the USA (Sowing. Carolina), Vietnam.