The idea that some gemstones are precious and others are only semi-precious is familiar to every buyer of colored stones. Precious stones – diamond, sapphire, emerald, and ruby – traditionally command a premium price in the market due to their extraordinary color or brilliance and extreme rarity.
One common misconception is that the distinction between precious and semi-precious gemstones is traditional, going back many centuries. In fact it is a recent innovation, dating only to the nineteenth century. The first use of semi-precious to mean “of less commercial value than a precious stone” can be traced back only to 1858.
The treatment and enhancement of gemstones has existed for hundreds and hundreds of years. The first documentation of treatments was presented by Pliny the Elder. And, 2000 years later, many of these treatments are still being used today! Some enhancements improve on nature, cannot be detected and are permanent; this provides the gem market with a larger supply of beautiful gemstones. Other treatments produce dramatic changes in the gemstone itself or its clarity; the irradiation and heating of colorless topaz that permanently transforms it into blue topaz is an excellent example.
Natural colored gemstones come from the earth, where combinations of heat, pressure, water, chemistry, and time create unique mineral deposits. Most colored gemstones are born in the earth’s top-most layer or “crust.” Peridots (like diamonds) are an exception, coming from the mantle, below.
Once formed, colored gemstones can be embedded in solid rock or moved by wind or water to more accessible secondary locations, like streambeds, and scattered in smaller pieces. Depending on the setting, the stones are recovered by machine or by hand and then, typically, cut and polished.
Colored gemstones are created all over the world, except Antarctica. While many types of stones can be found in multiple locations, others are more limited. Sapphires, for example, come from sites ranging from Madagascar to Montana. But, tanzanites are from a single, remote spot in Africa.
For any stone, the term “natural” refers to its original creation. It is quite common — sometimes essential — for a stone’s appearance (i.e. color or clarity) to be enhanced after the fact, by one or more sophisticated treatments. Untreated stones are actually quite rare.
In gemological terms, types of colored gemstones — such as emeralds, rubies, or sapphires — are called varieties. There are many varieties, and each one contains stones with specific, similar traits. Among other things, these can include chemical, mineral, and structural features, as well as aspects of durability, density, and light separation. These aspects are more formally known as hardness (on the Mohs Scale), specific gravity, and refractive index.
Emeralds, for example, are hexagonal structures made of beryl, or beryllium aluminum silicate. They are rated 7.5–8 on the Mohs Scale, with a specific gravity of 2.67–2.78 and a refractive index of about 1.577–1.583. Emeralds are green, courtesy of chromium, and sometimes, vanadium.
Rubies, on the other hand, are hexagonal crystal structures made of corundum, or aluminum oxide. They are harder and denser than emeralds (9 on Mohs, with a specific gravity of about 4.0). And they have a refractive index of approximately 1.762–1.770. Rubies are red in color, due to the presence of chromium.
Sapphires are also made of corundum and share the hardness, density, and refractive index of rubies. However, depending on trace mineral content, sapphires can exist in virtually any color.
Many other colored gemstone varieties also come in a surprising range of hues. Some of the most famous and the chemisttry of their colors appear below: