Spinel is not a single mineral, but it is used as the name for a series of minerals that contain varying amounts of magnesian spinel with iron, zinc and manganese, namely hercynite, and galaxite. Chromite may also be present. Gem-quality stones have long been prized, especially the red ruby spinel. Often mistaken for rubies and sapphires, spinel was collected in royal treasuries. The best-known stone is the Timur Ruby in the British Crown. Many spinels, like corundum, are now manufactured synthetically in large quantities. Red spinel turns brown or black on heating, changing to green, colourless and back to red on cooling.
Beryl is one of the oldest known minerals. The most prized variety emerald sometimes fetches higher prices than diamond. Emerald was mined in Upper Egypt in about 2000 BC. Queen Cleopatra is supposed to have owned an emerald on which her portrait was engraved. The short-sighter Roman emperor Nero may have used an emerald as a monocle, but historians now think that it was probably made of the pale blue variety aquamarine. Most blue aquamarines used in jewellery were originally greenish-yellow. The colour changes when they are heated to 400-4500C and then cooled. Some beryl crystals are enormous. One discovered in Albany, Maine, was 6 m long.
Tourmaline can show a variety of colours, even in a single crystal. The layers of the crystal may be arranged concentrically, parallel to the vertical axis of the crystal, or in longitudinal bands. Some crystals are strongly pleochroic to the naked eye and are used as gemstones. Other change colour when moved from natural to artificial light. Red tourmaline, introduced to Europe from Sri Lanka in 1703, was found to have the special property of attracting tiny ash particles on heating. These were the first observations of pyroelectricity.
This widely distributed form of silica is constantly used in jewellery and for ornaments. Rock crystal is water-clear. Milky quartz is white. Colours are caused by impurities, such as ferrum in violet amethyst, manganese or titanium in rose quartz, and iron hydrates in yellow citrine. Tiger’s eye is formed by fibrous inclusions of crocidolite. Tiny tourmaline, rutile and zoisite inclusions give blue quartz. Rutilated quartz contains needle-like yellow and red rutile crystals. Avаnturine quartz contains scales of mica or goethite, giving a spangled green or brown look. Quartz is used in glass-making, ceramics, refractories, building materials and abrasives. Its piezoelectric property makes it useful in pressure-sensitive devices in different radio and electrical equipment.
Colour. For coloured gemstones, the criteria are hue, colour saturation, brightness and colour purity (i.e. freedom from contaminating colours). The criterion used for the colour grading and series of polished diamonds is the degree of freedom from colour when viewed under specified conditions. Polished diamonds and gemstones are visually graded for colour by inspecting them through the side of the pavilion against a neutral white background (i.e. in a folded white grading card or a grading tray).
Clarity is a classification term in the grading of polished gemstones (particularly diamonds) that denotes the degree of freedom from those features such as inclusions, which would inhibit the passage of light through the stone. The term quality is similarly used in the sorting of rough diamonds.
Refractive indices (R.I.) are a measure of the degree by which a material bends or refracts the light rays entering it from the surrounding medium (air) at angles other than the normal. The refractive index of a material can be expressed as the ratio between its optical density and that of air. R.I. can also be expressed as the ratio of the velocity of light in air to that in the material. In doubly-refracting uniaxial crystals there are the maximum value index and minimum value index (ordinary ray and extraordinary ray).
Birefringence. When a light ray enters a gem belonging to any crystal system but the cubic one, it is split into two rays, which are plane polarized at right angles to each other. These two rays travel through the gem at different velocities and are therefore refracted by different amounts. Gems, which cause light to split into two polarized rays, are called doubly-refracting, birefringent or anisotropic.
Optic sign. The convention of designating birefringent crystals as optically positive or negative is based on the following relative values of the two refractive indices as measured on a critical-angle refractometer. With uniaxial stones, if, on rotation of the stone, the moving extraordinary ray R.I. is greater in value than the fixed ordinary ray R.I., the gem is optically positive. If the ordinary ray R.I. is greater than the extraordinary ray R.I., the gem is optically negative. With biaxial stones, if rotation causes the higher R.I. index to move more than halfway from its highest-reading position towards the lowest-reading position of the other index, the gem is optically positive. If the lower-reading R.I. moves more than halfway towards the higher R.I. the stone is optically negative.
Pleochroism. It is an optical property possessed by some coloured doubly-refracting gemstones in which the two refracted rays undergo differential selective absorption and emerge differing in colour or in depth of colour. In uniaxial stones two colours or shades can occur (dichroism). And in biaxial stones three colours or shades may be seen (trichroism).
6.4.Find Russian equivalents for the following words:
diameter of the sphere
6.5. Translate into Russian the following word combinations:
1. the degree of freedom
2. selective absorption
3. depth of colour
4. the cat’s eye effect
5. four-pointed star
6. precious opal
7. weak reddish fluorescence
6.6. Answer the questions:
1. What criteria of colour are for gemstones?
2. How is it possible to estimate colour of gemstones?
3. What is clarity of gemstone meant?
4. How refractive indexes are expressed?
5. What crystal system of mineral isn’t doubly-refracting stone?
6. Has biaxial mineral the optic sign?
7. What minerals possess intense pleochroism?
6.7. Translate into English the following sentences:
1. Чистота является условием качества ювелирных камней и алмазов.
2. Показатель преломления зависит от угла между преломленным лучом и нормалью.
3. Луч света преломляется, когда входит из воздуха в каменный материал.
4. Двупреломлением обладают все анизотропные минералы.
5. Кристаллы могут быть оптически положительными или отрицательными.
6. Плеохроизмом обладают некоторые окрашенные анизотропные камни.
Optical phenomenons of gemstones
There are optical effects created by light rays reflected back from beneath the surface of a gemstone.
Chatoyancy – the cat’s eye effect caused by the reflection of light from parallel groups of fibres, crystals or channels within the stone (e.g. as in the tiger’s eye variety of quartz and the cymophane variety of chrysoberyl).
Asterism – a star effect present in a few gemstones such as ruby and sapphire (which are polished as cabochons to show the effect to best advantage). Asterism is caused by sets of fine parallel fibres or crystals, which have aligned themselves along the crystals axes (in star corundum there are three sets intersecting at 600). In star diopside and some garnets a four-pointed star is produced as the result of two sets of inclusions intersecting at approximately right angles.
Iridescence – the ‘play’ of rainbow-coloured light caused by extremely small regular structures beneath the surface of the gemstone, which ‘interfere’ with the reflected light. In precious opal, iridescence is caused by millions of microscopic spheres of cristobalite that colour the reflected light by a combination of interference and diffraction effects.
Labradorescence – a particular form of iridescence seen in the labradorite and spectrolite varieties of feldspar that is caused by thin layers or flakes beneath the surface.
Adularescence – also known as Schiller and opalescence, this is the bluish iridescence seen in moonstone, and is caused by lamellar twinning.