Buying Guide: Gemstones
Gemstones are a little more complicated than diamonds. Diamonds have accepted grades for color and clarity and so it is possible for a price list like the Rapaport Diamond Report to exist. Gemstones have no grading system, each variety has individual value factors, and within each gem variety, quality dramatically affects price.
Like diamonds, gemstone quality and value are evaluated according to the “four Cs”: color, clarity, cut, and carat weight. Country or origin may also affect a gems’ value. Colored gemstones are also commonly treated, so that also affects value.
Color is the key factor. But don’t assume that the darker the color, the better the stone. Color can be too dark. Generally, the more bright and vivid the color, the better. Pure, clear, medium-tones are the most preferred.
The second most important factor affecting value is clarity. Clear transparent gemstones with no visible flaws are the most valued. There is no standardized grading system for clarity because it varies by gem variety. With colored gemstones, if the inclusion doesn’t show in the face up position, it generally doesn’t matter at all. Some varieties, notably emerald and red tourmaline, are very rare without inclusions of some kind so the price structure takes this into account.
In rare cases, inclusions can increase value. Special effects like the star in star sapphire and the eye in cat’s-eye chrysoberyl are caused by inclusions. Inclusions can also be a birthmark, proving that a gemstone is from a particular place.
Gemstones are sold by weight, not by size. Prices are calculated per carat, which is one-fifth of a gram. Some gems are denser than others so the same weight stone may be a different size. For example a one-carat emerald is bigger than a one-carat ruby. Just like diamonds, the carat weight also affects the price.
A good cut is something that may not cost more but can add or subtract a lot of beauty. A well-cut faceted gemstone reflects light back evenly across its surface area when held face up. If the stone is too deep and narrow, areas will be dark. If it is too shallow and wide, parts of the stone will be washed out and lifeless. The best way to judge cut is to look at similar gemstones next to each other.
Colored gemstones come in lots of different shapes, many more than diamonds. Choose whichever style appeals to you.
Country of origin matters in the prices of high-end ruby and sapphire but it doesn’t have to matter to you. Just know that one stone can cost more than an identical stone without confirmed origin. If you are buying a larger or premium gem then you need to think about its origin. If not, don’t worry about it other than the fact that it is kind of cool to know where a gem is from.
A few things you need to know about origin if you are thinking of paying for one of these premium gemstones:
- GIA doesn’t grade origin. The major labs that do so are AGTA in New York and American Gemological Laboratories in New York.
- Origin is guesswork. Only some stones show evidence and geology doesn’t respect national boundaries.
Most gemstones are treated. Garnet, peridot, iolite, spinel, or alexandrite are basically the only gemstones that aren’t. That being said, the trade distinguishes between good treatments and bad treatments. Trade-accepted treatments are the basic ones that everyone expects to have happen. Taboo treatments are “special” products that not everyone will carry or buy because they may not hold value as well as normal products.
- heating ruby and sapphire
- heating amethyst, aquamarine, citrine, tanzanite, tourmaline, precious topaz, and zircon
- irradiating blue topaz
- glass filling ruby
- diffusion treatment of sapphire
- epoxy resin in emerald
- dyeing opal, lapis, or pearl