Introduction to Common Precious Stones

The world of gemstones is beautiful and varied. While every jewel is highly valued by someone, we instinctively recognize the classic precious stones.

Amber is beautiful, and yet it’s obvious that no one would make a Diamond Room or Ruby Room as they once did an Amber Room. Tsavorite is a green garnet and possibly the rarest gemstone in the world today, but the Gemological Institute of America says that emerald’s history makes it more in demand and thus more valuable. Lapis lazuli has been called “the most expensive blue of all time,” but it was the Star of Bombay, a sapphire, that silent film star Douglas Fairbanks chose to give his wife Mary Pickford, who later donated it to the Smithsonian.

♦ The basics

What we call precious stones are really mineral crystals. Diamond is pure carbon and the hardest naturally formed mineral on Earth, coming in at 10 on the Mohs scale. A lapidary needs special tools to work on diamonds, so for this reason and others having to do with supply and economics, gemologists have established two classes for all gems:  diamonds and colored stones.

Rubies and sapphires are varieties of corundum, a mixture of aluminum and oxygen that, at 9 on the Mohs scale, is the second only to diamond in hardness.

Their hardness is one very good reason why diamonds, rubies and sapphires have become precious over time. It not only enables them to withstand much handling down through the centuries, it also makes them easier to find because they resist the weathering that erodes surrounding rock formations and brings the rough gems closer to the surface.

Pure corundum, like diamond, is colorless, but if it contains ions of the metal chromium, the mineral can absorb all wavelengths of light except red, which it reflects – a ruby has formed.

Things get a little more quantum if the corundum picks up some iron and titanium ions that can exchange electrons through the very unromantic-sounding intervalence charge transfer process. The exchange requires energy, specifically that with a wavelength corresponding to that of yellow light. When you subtract this from ordinary white light, a blue color results – the famous deep blue of precious sapphires.

Emeralds aren’t as hard as the other precious stones, only around 7.5 to 8 on the Mohs scale. Made of beryl – a mineral made of beryllium, aluminum, silicon and oxygen – they often have inclusions and fractures. Chromium, the same thing that makes rubies red, makes emeralds green. The internal geometry of beryl is slightly different from corundum, resulting in a stone that absorbs all wavelengths except green. Since the discovery of a large deposit of vanadium-colored stones in 1963, beryl with vanadium in it has also been classified as emerald.

♦ The 4Cs

It doesn’t require a science degree to walk into a jewelry store to look at precious stones, but it can be difficult to understand the value of what you will be offered there.

Gem color is one of the 4Cs, a gem grading system that was established by the Gemological Institute of America in the first half of the 20th century and today is used throughout the world. While most often publicized for diamonds, the 4Cs are also used to evaluate rubies, sapphires and emeralds. Clarity, cut and carat weight are the other three Cs.

♦ Color

In addition to the hues of red, green or blue, a gemologist will also look at saturation, that is, whether the hue is strong or pastel. Light or dark tone is also important. Rubies should be a strong red with a medium to medium dark tone. The most valuable sapphire color, Kashmir blue, has an intense, dark tone. A strong, dark green is considered ideal for emeralds.

Diamond color grading is more complicated, with three grades of transparency (grades D-F) and four grades of white (grades G-J), although all will appear colorless in a setting. After those come the grades of K to Z, where tint, usually yellow or brown, increases and the gems decrease in value until the tinting gets dark in tone, giving the stone value as a fancy.

♦ Clarity

Gemstones usually develop inclusions during their formation.  Inclusions may be small pieces of material, hollow spaces or fractures; they all block the passage of light through the stone to a greater or lesser degree.

Colored stone clarity is usually “clean” (no inclusions visible to the naked eye) or slightly, moderately or heavily included. The clarity of emeralds is graded a little more tolerantly than the other precious stones, because they usually do have inclusions. “Some actually prefer Emeralds with minute flaws over flawless Emeralds, as this proves authenticity,” notes Hershel Friedman at Experts can also sometimes check authenticity by looking for the rare three-phase inclusion of solids, liquids, and gas that is typical of many authentic Colombian emeralds.

Diamond clarity has many grades. A flawless diamond is one where no inclusion can be seen at 10x magnification. After that, clarity grades are given using combinations of the letters V (very), S (small) and I (inclusion). Only microscopes can detect the inclusions in VVSI1, VVSI2, VSI1, VSI2, SI1 and some SI2 diamonds, but those letters will make a big difference in price. Diamonds of I1 and I2 clarity, as well as some SI2 stones, have inclusions that can be seen without a microscope, but these are still considered to be gems.

♦ Cut

Diamond’s colorful sparkle derives from its physical properties of refraction and dispersion, but it takes a skilled gem cutter to release the fire in each stone.

Indeed, all precious stones have had their surfaces specially cut and polished. The famous octagonal “emerald cut,” for example, was originally developed because all those inclusions, combined with its slightly lower hardness, make emeralds more susceptible to damage during cutting.

The details of this precision work are complex, but there are some easy ways to get an idea of how well a gem has been cut. Look for symmetry in the overall shape of the stone, and a high degree of light return. The center of the stone should be as dark as the rest of the gem and have the same sparkle.

Don’t hesitate to ask your jeweler for a look at it under magnification. The jewel’s reflective surfaces should be flat and shiny, without pitting or scratches. Facet joints should be sharp and come together in a single point.

♦ Carat weight

The size of precious stones is measured in carats.  Once calibrated to the mass of a carob seed, according to legend, today’s carat equals 200 mg. Large stones are rare and therefore cost more per carat. On a diamond broker’s price list, for instance, a 0.50 carat flawless diamond might cost 3 times as much as a 0.25 carat flawless diamond of the same color and clarity.

♦ Treatments and synthetic gems

For centuries, people have been treating precious stones to improve their appearance. Some treatments, like irradiation of diamonds to produce color, or laser drilling them to remove inclusions, are common. An estimated 95% of rubies undergo some sort of heat treatment. Emeralds are routinely treated with colorless oils or resins; however, experts note that high-quality unenhanced emeralds, if certified, can be up to 50% more valuable than the equivalent treated stones.

High-quality artificial diamonds, rubies, sapphires and emeralds, as well as other gemstones, are made today and more and more are being worn for their own sake rather than just as inexpensive substitutes for the real thing. Your jeweler can help you decide which type of precious stone, natural or synthetic, is right for you.

All around us, gemstones glitter and gleam, but diamonds, rubies, emeralds and sapphires are still something special. These classic precious stones fit into our modern world as beautifully as they have done for thousands of years, in civilizations all over the world.