Colored Gem Stones
What could be more tempting than a jewelry display packed with a rainbow of brightly colored gemstones? Go shopping and you'll see natural, synthetic, and imitation stones of every color, shape, and size. Can you tell one type of gemstone from another? Here are some tips to help you understand the differences so you can ask the right questions before you buy a colored gemstone.
Natural Colored Gemstones
Natural stones are courtesy of nature, with no interference from humans. Don't assume that just because it's natural a stone should carry a high price tag. Prices are driven by desirability, quality, and availability. A brilliantly colored ruby with "perfect" clarity will cost thousands of dollars more than a garnet of similar quality. Become acquainted with the gemstone market before you buy.
The traditional classification in the West, which goes back to the Ancient Greeks, begins with a distinction between precious and semi-precious stones; similar distinctions are made in other cultures. In modern usage the precious stones are diamond, ruby, sapphire and emerald, with all other gemstones being semi-precious.
This distinction is unscientific and reflects the rarity of the respective stones in ancient times, as well as their quality: all are translucent with fine color in their purest forms, except for the colorless diamond, and very hard, with hardness of 8-10 on the Mohs scale.
Other stones are classified by their color, translucency and hardness. The traditional distinction does not necessarily reflect modern values, for example, while garnets are relatively inexpensive, a green garnet called Tsavorite, can be far more valuable than a mid-quality emerald.
In modern times gemstones are identified by gemologists, who describe gems and their characteristics using technical terminology specific to the field of gemology. The first characteristic a gemologist uses to identify a gemstone is its chemical composition. For example, diamonds are made of carbon (C) and rubies of aluminum oxide (Al2O3). Next, many gems are crystals which are classified by their crystal system such as cubic or trigonal or monoclinic. Another term used is habit, the form the gem is usually found in. For example diamonds, which have a cubic crystal system, are often found as octahedrons.
Gemstones are classified into different groups, species, and varieties. For example, ruby is the red variety of the species corundum, while any other color of corundum is considered sapphire. Emerald (green), aquamarine (blue), red beryl (red), goshenite (colorless), heliodor (yellow), and morganite (pink) are all varieties of the mineral species beryl.
Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster. They may exhibit pleochroism or double refraction. They may have luminescence and a distinctive absorption spectrum.
Material or flaws within a stone may be present as inclusions.
Most natural stones are treated to improve appearance. Heat and radiation change or enhance colors. Diffusion deepens color, but only within a stone's outer layers. Oil and waxes are used to fill-in surface-breaking fractures. Some treatments are permanent--others are not. Treated gems can be a good choice when you know what you are buying and pay a price that reflects a stone's true quality.
Synthetic Colored Gemstones
A synthetic stone shares a natural stone's physical, chemical, and optical qualities. The difference? Synthetics are created in a lab. They've been around for a long time, but modern technology allows us to grow stones that are difficult to distinguish from their natural counterparts. Ask for a lab certificate to verify authenticity before paying top dollar for a stone represented as natural.