There are over 3000 different minerals, out of these only 50 are commonly used as gemstones. To be regarded as a gemstone the mineral must be beautiful particularly in colour and it must be rare because this increases the value. A gemstone must also be durable and hardwearing enough to survive constant use without becoming damaged.
Gemstones are generally made up of minerals which are natural, inorganic materials with a fixed chemical composition and a regular structure internally. A few gems are organic; these are produced by living organisms such as shellfish. Amber and pearls are two examples of an organic gemstone. Another type of gemstones are called synthetics which are not natural and are made in laboratories, these are often used as imitations of the real thing.
Gemstones begin their life as a crystal as shown with the amethyst to the left. This is normally built up of crystalline with their atoms arranged in regular and symmetrical patterns, like a lattice. This crystalline is normally embedded in a host rock known as the matrix. In this state the stone is referred to as rough. In its natural state the mineral may be a prism with clearly defined faces as shown with the hexagonal crystal on the right.
The crystals are faceted and polished to make them more attractive and to be set in a piece of jewellery or an ornament. Faceting is the most usual method of fashioning a gem. It is where the surface of the gem is cut in phases. A good example of this a brilliant cut diamond. The diamond begins as a rough stone; the first stage is to cut the top off and to round to lathe of the stone with another diamond. Next the central facet, the flat table, is ground then the bezel facets. Four more sets of facets are then added before the “brillianteer” adds a further 24 facets above the girdle and 16 below. The stone is then polished and this will give the stone a beautiful sparkle. There are four common cuts of diamonds, brilliant cut, step cut, mixed cut and fancy cut.
Sometimes crystals are polished instead of being faceted; this is the oldest form of fashioning a stone. Organic gemstones are most commonly fashioned in this way as they are usually translucent or opaque meaning there would be no advantage of faceting them. Gem fragments of similar hardness are turned into attractive pebbles by tumbling them in a drum containing abrasive grits and polishing powders. It is a similar process to what naturally happens to stones on beaches as they are worn against each other, they eventually become smooth. The most common shape that a stone is polished into is called a cabochon which is a simple cut used to display colours and optical effects in opaque and translucent stones. This is demonstrated well with star sapphires.
In many stones there are natural inclusions, these are internal features in gemstones. These may be solids, liquids or gases that the crystal has enclosed whilst it grew. Sometimes they are also cracks or fractures that have filled after it has finished growing. These are often regarded as flaws or inclusions in the stone; however they do add extra interest and can be helpful when trying to identify the stone. They can also help to create a more attractive stone for example a star sapphires are created by needle inclusions in the stone which have formed parallel to the crystal faces. Another interesting example of an inclusion is when a of a different gemstone forms inside, and example of this is shown right with two small garnets that have formed in a diamond This is not normally visible to the naked eye but does show as a slight mark inside the diamond.
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