What Makes A Mineral A Gemstone?

There are over two hundred recognized gemstones. Most are minerals and are cut from naturally occurring crystals. To qualify as a gem, the specimen should be hard and tough to resist scratching and have beautiful clarity, colour or fire. Value is based on rarity and rare quality.

Gemstone Colours

As mentioned earlier, gems appear coloured due to some of the white light being absorbed within the minerals structure. Most gems are coloured by a limited number of metals present either as impurities, as in Allochromatic gems or as essential constituents of the atomic structure, as in Idiochromatic gems. The most common minerals affecting colour are; Chromium, Iron, Manganese, Titanium and Copper.

RUBY FACET

It is Chromium which produces the intense red of Ruby and the brilliant green of Emerald. In Garnet, Spinel, Sapphire, Peridot and Chrysoberyl it is Iron that is responsible for the more subtle red, blue and yellow colours. The best Sapphires are coloured by both Titanium and Iron whilst copper is responsible for the blue and green of Turquoise and Malachite, and Manganes for the pink of Rhodonite.

Some minerals give us more than one gem type. This is because the impurity, which is only a minute amount, is not always the same and may even be a combination of impurities. The interaction of the impurities and the atomic structure will also have an effect on colour.

For example, the mineral Corundum, which is Aluminium Oxide, is colourless in its pure form, whereas if it contains Chromium it becomes the rich red of Ruby, if it contains Titanium it becomes the intense blue of Sapphire. Aquamarine and Emerald are different colours of the mineral Beryl, which can also be orange, yellow and green.

Bi-Coloured Quartz
Amethyst/Citrine

Quartz displays a profusion of colours, patterns and optical effects unsurpassed by any other gem – purple Quartz is Amethyst, yellow is Citrine, brown is Smoky Quartz (Cairngorm) and pink is Rose Quartz. Agate, Tiger Eye and Aventurine are some of the Quartz gems that display many unusual effects and colours.

Optical Effects In Gemstones

DISPERSION

The brilliant colours displayed by Opal and Labradorite and the sparkle of Diamond are optical effects caused when white light is split into its constituent colours. In Diamond especially, and in many other transparent gems, the ‘Fire’ and sparkle is caused by dispersion as the gemstone acts like a prism.

In Opal, this is caused by the light being scattered as it passes through planes of regularly spaced voids between the transparent silica spheres. The colours are the result of interference and the range of colours depends on the distance between the planes.

Cutting & Carving Gemstones

The art and practice of gemstone cutting is known as lapidary. With consummate skill, the lapidarist can turn a rough mineral fragment or water worn pebble into a sparkling and valuable gem. The techniques used have been developed over many centuries and these skills, when combined with modern techniques allow gems to be cut to obtain the very best effects from each mineral’s inherent attributes or special optical characteristics.

CABOCHON CUT TURQUOISE

Different cutting styles suit different gems. Transparent specimens like diamond are best faceted whilst the opaque gems, Agate, Onyx, Turquoise and Malachite are best cut as cabochons or as beads. The word Cabochon is a French word derived from the Latin ‘cabo’ meaning head or dome. Cabochons are the oldest and simplest way of cutting gems and the technique is still popular to this day.

ROCK CRYSTAL CARVING

Larger pieces of gem material such as Jade are often carved, though it would be more correct to say ground, into figurines, animals, bowls, vases and other artistic forms. The faceting of gemstones was developed much later than carving and did not become commonplace until mediaeval times.
The techniques were honed to their full potential during the last century.

Crown Facets

Pavilion Facets
STANDARD BRILLIANT

Today, we understand more about the physical and optical properties of gems and we are able to measure the properties and how light  affect them. The modern lapidary with this knowledge and the use of very sophisticated and accurate machines can cut more beautiful gems than ever before. The gem is worked into a pattern of highly polished flat planes or surfaces which we call facets and which act as mirrors. Some light reflects from the top crown facets and displays the gem’s lustre. Light also enters the gem through the crown facets and this is reflected off the bottom pavilion facets to re-emerge through the crown.