Geology Of Gemstones - Resources Information about Gemstones

How Gemstones Are Made

A gemstone is a mineral, rock (as in lapis lazuli) or petrified material that when cut or faceted and polished is collectable or can be used in jewellery. Others are organic, such as amber (fossilized tree resin) and jet (a form of coal). Some beautiful gemstones are too soft or too fragile to be used in jewellery, but are exhibited in museums and are sought by collectors of mineral or crystal specimens.

There are three types of gemstones: Natural, Synthetic, and Simulant.

Natural gemstones include emeralds, rubies, sapphires, amber, coral, fossil, ivory, cultured freshwater pearls and natural saltwater pearls. A natural gemstone is a mineral, stone, or organic matter that can be cut and polished or otherwise treated for use as jewellery or other ornament. A precious gemstone has beauty, durability, and rarity, whereas a semiprecious gemstone has only one or two of these qualities. A gem is a gemstone that has been cut and polished. Diamond, corundum (ruby and sapphire), beryl (emerald and aquamarine), topaz, and opal are generally classed as precious stones. All other gemstones are usually classed as semiprecious.
Natural gemstones are simply mineral crystals whose chemistry and structure make them look special.

Synthetic and Simulant are terms used for laboratory grown gemstones.
Laboratory grown synthetic gemstones have essentially the same appearance and optical, physical, and chemical properties as the natural material that they represent. Synthetic gemstones include alexandrite, coral, diamond, emerald, garnet, lapis lazuli, quartz, ruby, sapphire, spinel, and turquoise.

Laboratory grown simulants copy the look and colour of the real stone but possess neither their optical, chemical nor physical characteristics. The Gemstone Simulants include coral, cubic zirconia, lapis lazuli, malachite, and turquoise. Additionally, certain colours of synthetic sapphire and spinel, used to represent other gemstones, would be classified as simulants. Coloured and colourless varieties of cubic zirconia are the major simulants produced. In the past few years, the use and consumer acceptance of synthetic and simulant gemstones has grown. Much of this growth is the direct result of the recognition of these gemstones for their own merits, not just as inexpensive substitutes for natural gemstones.

However, true synthetic gemstones are not necessarily imitation. For example, diamond, ruby, sapphire and emerald have been manufactured in labs, which possess very nearly identical chemical and physical characteristics as the genuine article. Synthetic corundums, including ruby and sapphire, are very common and they cost only a fraction of the natural stones. Smaller synthetic diamonds have been manufactured in large quantities as industrial abrasives for many years. Only recently, larger synthetic diamonds of gemstone quality, especially of the coloured variety, have been manufactured.

Gemstones are rare, and when present at all, tend to be scattered sparsely throughout a large body of rock or to have crystallized as small aggregates or fill veins and small cavities. Even stream gravel concentrations tend to be small - a few stones in each of several bedrock cracks, potholes, or gravel lenses in a stream bed.

Gemstones occur in most major geologic environments.
Each environment tends to have a characteristic suite of gem materials, but many kinds of gems occur in more than one environment. Most gemstones are found in igneous rocks and alluvial gravels, but sedimentary and metamorphic rocks may also contain gem materials. Diamonds are precious, lustrous gemstones made of highly-compressed carbon.

Examples of geologic environments in which gemstones are found include:

Pegmatite - a coarse-grained intrusive igneous rock body, occurring as dikes (a tabular-shaped body), lenses, or veins in the surrounding rock;

Stream gravels (placers) - deposits of heavier and more durable than average minerals that have been eroded out of the original rock. Often tourmaline, beryl, and many other gem-quality minerals have eroded out of the original rock in which they formed and have moved and been concentrated locally by water in streams;

Metamorphic rocks - rocks that have been altered by great heat, pressure, or both. Garnet, for example, is commonly found as crystals in gneiss and mica schist.

Gemstone pebbles are made by tumbling rough rock with abrasive grit, in a rotating drum.

Of the 2,000 minerals identified in the world, only about 16 yield important gemstones. To be a gemstone, a mineral has to be beautifully coloured, hard, quite durable, and, most of all, rare.

The colour of any gemstone is due to the nature of light itself. Sunlight, often called white light, is actually a mixture of different colours of light. When light passes through a gemstone, some of the light may be absorbed, while the rest passes through. The part that isn't absorbed reaches our eyes as white light minus the absorbed colours. A ruby appears red because it absorbs all the other colours of white light - blue, yellow, green, etc. - and reflects the red light to the viewer. A colourless stone absorbs none of the light, and so it allows the white light to emerge unchanged.

Most raw gems (including diamonds) have a rough shape and a dull colour. Only after they have been cut and polished do they take on that special glow that people have come to expect. A person who cuts gems is called a lapidary, but actually that person does very little cutting. Instead, a lapidary uses a variety of grinding wheels and grits to shape and polish the coloured gems or stones.
Gems are usually cut to highlight their internal colour or natural crystal shape. The two main cutting techniques produce either cabochons or faceted gemstones. Cabochons are stones that have been cut, ground into the shape of a dome, and then polished on the outer surface. This technique is used primarily for opaque stones like opals that don't let the light shine through.
Faceting is generally used with gems that are transparent. By grinding regular, flat surfaces in a predetermined geometric pattern on the outside of the gem, a lapidary turns a rough diamond stone into a brilliantly-sparkling gem.
When light enters a faceted gemstone, it is bent to a different angle. This is called refraction. The facets on the outside of the gemstone are positioned so that the light enters the stone from the top, is bent, and eventually is reflected back to the viewer, displaying the brilliance within the gem. Nature provides the gemstones, but it's human interference that turns them into jewels.

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