About Gemstones
Agate is the name given to a wide selection of varieties of chalcedony, a form of quartz . Microscopically small crystals of quartz are embedded in opal, which is itself hydrated quartz. The proportion of quartz to opal varies, and this variation is reflected in differing densities, refractive indexes and other properties. The quartz crystals are in a fibrous form, sometimes running approximately parallel to each other, sometimes entwined.
The names for chalcedony and agate are somewhat interchangeable, although the term agate is usually used to describe the banded varieties of chalcedony. Many agates are naturally coloured, others are stained for decorative effect. Agate can be many different colours, and because its banding is so variable, different types of it have been given many different descriptive names. It is often used to make quite large decorative items. Some names include moss agate and picture agate. Many types of organic material have been fossilised by agate, where the organic material has been replaced slowly over a long period of time, so that the original structure of the original item is retained. Fossilised or petrified wood is an attractive form of agate.
Amazonite or Amazonstone
Amazonite is a gem variety of microcline felspar, being potassium aluminium silicate. It is usually polished as a cabochon. In common with other feldspar gems, it displays a schiller of coloured light, caused by inclusions or its crystal structure creating interference effects of light. It is sometimes called amazonstone possibly because one of its sources is Brazil, although not from the Amazon regions. Amazonite varies from bright green to a bluish green
Amber
Amber is one of the organic gemstones, being the time-hardened fossilized resin of pine trees. As such it varies from about 20 to 60 million years old, according to different sources. It appears surprisingly light and warm to the touch, and readily produces static electricity when rubbed.
Amber is most usually similar colours to honey, varying from golden yellow, through rich orange and red to brown, but some can be white, dark brown almost to black, blue or green. The best amber is usually clear and translucent, but inclusions can enhance its beauty, rarity and value. Amber can be treated or processed in a number of different ways. The most important source of Amber is from the Baltic area. It is found in two sources, from the sea and by mining. The sea amber is easily carried by the sea, and can also be found in all parts of the Baltic coast, even as far away as Norway, Denmark and the east coast of Britain. There are many other sources of amber, usually with different characteristics and colours, and often regarded as different varieties. Burmese amber is usually redder than Baltic amber and is known as burmite. It is harder, denser, and often contains calcite. Amber found in Sicily, along the Simeto river near Catania is reddish brown, fluorescent, and is known as simetite. Amber is found in many parts of Romania, is known as roumanite, and can be very variable in colour.
Amethyst
Amethyst is the clear purple, mauve or violet form of the mineral quartz. As such it is related to citrine which is the yellow form of quartz, and also to rock crystal which is the colourless variety. It is possible for some specimens of quartz to be different colours in different areas. Amethyst and citrine are varieties of quartz which can both occur in the same stone. Such stones are sometimes called ametrines. Amethyst gets its name from a Greek word amethustos meaning "not drunken", as it was believed to protect against intoxication by alcohol. We believe that amethyst is one of the most beautiful of the coloured gemstones, particularly in its better qualities. As with other gemstones, the "best" colour can vary according to personal preference, and the quality of amethysts can vary considerably. The most important attribute should be an attractive colour. The rich deep violet colour is generally the favourite and most expensive, but a stone of medium colour intensity, with plenty of sparkle, can also be very attractive.
Cornelian
Cornelian is a red variety of chalcedony which is cryptocrystalline quartz. Its red colour is due to the presence of iron impurities in the form of iron oxide or hematite. It can vary from a flesh red to a clear red. It is usually cut as cabochons, or into beads. Originally found in the deserts of Arabia and Egypt, it has also been found in the Rio Grande area, India, China, Colombia, Saxony, Scotland, West Germany and USA.
Citrine
Citrine is the clear yellow or golden form of the mineral quartz. The yellow color is from the presence of iron, the darker the color - the higher the grade.
As such it is related to amethyst which is the purple form of quartz, and also to rock crystal which is the colourless variety. It is possible for some specimens of quartz to be different colours in different areas. Amethyst and citrine are varieties of quart which can both occur in the same stone. Such stones are sometimes called ametrines.
Garnet
Garnet is a naturally occurring gemstone. Its name comes from Latin granatus meaning seed, because it often Rather than a single gemstone, garnet is a family of related minerals, some of which occur as gemstones.
Each has a common crystal structure, and a similar chemical composition. The popular understanding of garnet is as an inexpensive dark red stone. Because it is relatively common and inexpensive, it is often thought of as "only garnet", and as being inferior. This bias extends to other rare and attractive forms of garnet. Garnet occurs naturally in a large range of colours including: red, orange, brown, green, yellow, and brown. Its variability of colour reflects the variations in its composition.
Haematite
Haematite is a compact form of iron oxide. It varies considerably in compactness and form. Its hardest and most compact form is what gets used in jewellery as a gemstone. This is black, and is normally found in massive form as nodules The compact black forms have specific gravities and hardness at the higher end of the range. The softer forms are used as red pigments, and as jeweller's rouge. Gem quality haematite is black, although some describe it as blue-black. One of the main sources is Cleator Moor in Cumbria, England, but it is also found in Elba, the Swiss alps, Germany, Scandinavia, and the USA.
Labradorite
Labradorite is a gem variety of plagioclase feldspar. It is usually polished as a cabochon. Its beauty as a gemstone is mainly because of its schiller which is caused by interference effects of light due to fine lamellae (layers or plates) formed by repeated twinning of its crystals, and also by the inclusion of tiny platelets of impurities such as magnetite, an iron oxide. The body colour of labradorite is grey, and the schiller is usually blue and green, but yellow, gold, red and purple are among the other iridescent colours displayed. The flashes of colour in labradorite have been compared to the wings of tropical butterflies.
Lapis Lazuli
Lapis Lazuli is a beautiful deep blue stone, although its exact colour varies because it is a mixture of minerals whose proportions vary. It is opaque and slightly soft, therefore is almost always cut en cabochon, or as beads, rather than being facetted. The word lapis is latin meaning stone. Although lapis, as it is often called, is deep blue, its colour does show variation depending on its source and exact composition. Some material is lighter blue or greenish blue, some a rich purple blue, almost black. The best colour is an intense deep blue. The pigment ultramarine used to be made from crushed lapis lazuli, and probably provides the best description of its most desirable colour. It usually contains golden coloured flecks of pyrites. These are normally regarded as increasing the desirability, and are an indication of authenticity. Lapis also contains calcite, which sometimes shows as white flecks. These flecks are regarded as decreasing the beauty and desirability of lapis, especially if they are large or numerous, although tiny flecks can help to indicate that it is genuine. The deep blue colour is mainly due to the presence of hauynite. Apart from its traditional source in Afghanistan, lapis lazuli is also found in Chile, Siberia, Burma, and California.
Moonstone
Moonstone is the best known gem variety of orthoclase feldspar, being potassium aluminium silicate. It is usually polished as a cabochon. Its importance as a gemstone arises because of its schiller, which in moonstone has been given the name adularescence. This is caused by a feature of its crystal structure. Orthoclase feldspar and albite are present in close association, arranged in layers. This causes an interference effect of light. The colour of this schiller regarded as most desirable is blue, which usually occurs in a white or grey body colour. If the albite layers are too thick this schiller appears whitish which is less attractive. Moonstone contains a number of typical inclusions which appear to be stress cracks. These give the appearance of centipede type insects, negative cavities looking like rectangular crystals, and needle-like shapes. If present in large numbers, the needle shaped inclusions can cause a cat's eye chatoyancy effect. The most desirable colour of moonstone is blue, but it also occurs in grey, white, pink, green and brown.
Peridot
Peridot is a bright yellow green or golden green variety of olivine. It was originally found on Egypt but is also found in Burma, Sri Lanka, USA and Norway. Because there hardness is lower than 7, they are not ideal for use in rings, and should be treated with reasonable care. Bright golden green, but can vary to darker green or greenish yellow. Peridot has also been known as Chrysolite, although this is an old name which was applied fairly indiscriminately to any yellow and greenish yellow stone.
It is actually very common, and occurs in many varieties, a number of which are gemstones, some of which are
Topaz
Topaz of the best known gemstones. Even its name sounds like something exotic and fabulous from The Arabian Nights, or poetic like "silken Samarkand". In fact its name is so popular that most of the owners of a citrine claim to own a topaz! The name is believed to have derived from the Greek work topazos, from the Sanskrit tapas meaning fire. Topaz is well known to be yellow, and in ancient times all yellow stones were called topaz. Nowadays we know better.
Topaz can also be colourless, blue, green, pink, orange or brown. In the last 10 years or so, jewellers' windows have become filled with blue topaz, which is very attractive and inexpensive, and has to some extent become a substitute for Ceylon sapphire. Blue topaz does occur naturally, but almost all commercially available blue topaz is produced from less attractive colours which are irradiated and heat treated to turn them blue. This treatment produces a stable colour. The most usual colours are known as "London Blue", "Swiss Blue" and "Sky Blue", we have listed these in order from the deepest to the palest colours.
Turquoise
Tuquoise is an opaque blue to greenish gemstone, a hydrous phosphate of aluminium, with some copper and iron. Although it's crystal structure is triclinic, it rarely forms large crystals, and is said to be cryptocrystalline. It is also usually found with patches of white matrix interspersed, which tends to be less attractive, and reduces its value. The colour turquoise is named after the gemstone. The best specimens are a beautiful light blue stone, with green overtones. In general, the more green the stone, the less attractive it is considered. Unfortunately, turquoise will tend to become greener with age, due to exposure to heat, light, dirt, grease, and chemical attack. Historically, most turquoise came from Persia, now Iran, which is still the source of most of the world's best turquoise. It is also found in Egypt, Turkestan, and the USA.
