Chromium Makes the Difference

By Gary Roskin, G.G., FGA, Senior Editor -- JCK Online, 8/1/2003 2:00:00 AM

If you see the label "chrome" attached to any gemstone, chances are the color will be saturated and pure, and the price will reflect the gem's exceptional color. But not every gem labeled "chrome" deserves the label or the price. You must see the difference in color to justify the price on the label.

Compare one green tourmaline to another and you'll see a difference in saturation and purity of color. The finest colored gems owe their beauty to chromium.

Color due to chromium is rich and pure because of how the chromium absorbs light. White light is made up of the rainbow of color wavelengths. We see color because the stone absorbs and transmits some of these wavelengths. The coloring agents in gems—typically metallic elements like iron, chromium, nickel, cobalt, titanium, vanadium, magnesium, manganese, etc.—absorb certain wavelengths, allowing the remaining wavelengths to be transmitted. The human eye perceives the transmitted wavelengths as color.

Chromium absorbs specific wavelengths and allows the transmission of great numbers of totally unaffected wavelengths. Other elements may absorb portions of a large number of wavelengths, giving the stone a less pure and more muted appearance.

Thus, gems that contain chromium have bright, saturated color—the most desirable type. Good examples of chrome-colored gems are Colombian emeralds, Burma red spinels, chrome diopsides, and Arizona chrome red pyrope garnets.