The most popular variety of quartz is amethyst, a transparent form whose color ranges from pale violet to deep purple. In many cut stones of amethyst the color intensity changes sharply from section to section. This is due to irregular color zoning common to amethyst crystals. The actual cause of the purple color in amethyst is not very well understood. There are fewer cut stones of amethyst in very large sizes because of the rarity of large, flawless, well-colored crystals.
This 4500-carat pale smoky quartz egg from California rests on a gold stand set with Montana sapphires. The unique gem was cut and its stand was designed and made by Capt. John Sinkankas as a difficult exercise in the lapidary art. The quartz egg is 4 inches long and almost 3 inches in diameter.
The name citrine (from the French word for lemon) attempts to describe the yellow color of another variety of quartz. Actually, the normal coloring of citrine varies from yellow to red-orange and red-brown, but the yellow sometimes rivals the yellow of topaz. In addition to the normal color range, the colors of citrine may grade through a grayish yellow variety known as cairngorm and a grayish variety called smoky quartz to a black variety called morion. Other varieties that add color dimensions to the group of quartz gemstones are rose quartz and milky quartz. Like amethyst, the reason for the color in rose quartz has not been definitely established. Milky quartz owes its color to myriads of tiny cavities containing water or liquid carbon dioxide.
A 783-carat step-cut citrine of deep, rich color dwarfs a 278-carat brilliant-cut citrine (at left), a 90-carat smoky quartz, and a 91-carat briolette of citrine. The smoky quartz, from Switzerland, is so dark that it appears to be opaque. The other three stones came from Brazil. The briolette and brilliant-cut citrines were cut and donated to the Smithsonian Institution by Albert R. Cutter. (Slightly less than half actual size.)
The range of color in quartz is somewhat surprising, considering that the mineral is a simple silicon dioxide. Some of the colors, as with corundum and some other gemstones, are due to traces of impurities. In quartz, these consist mainly of oxides of iron, manganese, and titanium. However, all the reasons for quartz coloration in its many varieties are not known.
Pastel rose quartz has a delicate beauty in any cut. The 375-carat step cut (top), the 84-carat step cut, and the 46-carat marquise came from Brazil. (Two-thirds actual size.)
