Mineral Guide > Gems > Color of Gems

Color of Gems

The color of gems is one of the most essential features of their value. While certain colorless gems, such as the diamond, are highly prized, even the diamond would lose much of its value if it did not flash colored lights.

So the quality of affording a permanent color probably leads to much of the esteem in which gems are held. The colors of the rose and the violet are not less pleasing than those of the ruby and amethyst, but the former endure but for a day, while the latter can be handed down unimpaired from generation to generation. It was probably to secure varieties of color that the ancients first used gems, for their classifications and designations of precious stones were based chiefly upon this property. With them almost any green stone was known as emerald, blue as sapphire, and red as ruby or carnelian.

This fact makes it difficult in reading accounts of gems as given by ancient authors to know what mineral is meant. Distinctions of hardness and specific gravity, now so much in use, seem to have been ignored by them for the most part. With the grouping of minerals according to their chemical composition, the significance of color largely disappeared as a means of distinction, since individual specimens of the same com-position, and hence the same species, may vary greatly in color. Usually the quantity of ingredient required to produce a certain color is too small to be detected by chemical analysis.

That the custom of distinguishing gems by their colors still survives, however, to a considerable extent, is evidenced by the fact that different names are still applied to gems of the same mineral when of different colors. Thus sapphire and ruby are both corundum; and emerald and aquamarine are beryl. The mineral quartz appears in a mul-titude of colors, to nearly all of which different names are given. Hence gems of two different names may occur even in the same crystal: as in a piece of quartz, from one portion an amethyst may be cut and from another a citrine. On the other hand, different species may present stones of exactly the same color. Thus corundum, spinel and garnet all afford red stones, often nearly alike in tint; or emerald and tourmaline both give green stones.

Speaking from the mineralogical standpoint, there are few minerals and fewer gems in which color is a constant and essential property. Those which may be mentioned as belonging to the latter class are pyrite, which is brass yellow, lapis lazuli, which is blue, and malachite, which is green.

In a few cases differences of chemical composition are indicated by differences of color. This is true of garnet, the magnesium-aluminum varieties of which are ruby red, the calcium-aluminum varieties brownish red, and the calcium-chromium varieties green. So tourmaline, when containing an excess of iron, is black; an excess of sodium and lithium is green or red, and an excess of magnesium is brown.

Usually, however, the coloring matter is foreign to the essential composition of the mineral, and of very small amount.
This coloring ingredient is in the majority of cases organic matter of some sort, chiefly hydrocarbons. This has been proved in some cases by analysis, and in general may be assumed when the color of a stone can be driven out or changed by heat. The following gems quite certainly owe their color wholly or in part to organic matter:-smoky quartz, amethyst, yellow topaz, golden beryl, zircon, rubellite, and amazon stone. The coloring ingredients of the following are chiefly inorganic: - ruby, sapphire, spinel, and emerald.

Next to organic matter metallic oxides are probably the most prevalent coloring ingredient. These oxides may occur in scales large enough to be seen with the naked eye, as is true of the hematite in sunstone, or they may be only visible with the microscope, as the same substance can be seen coloring jasper and carnelian. More commonly the coloring matter cannot be discerned as a distinct pigment. Beside oxide of iron as a coloring ingredient, chromium, copper and nickel oxides occur, producing in general green colors. Manganese oxide often gives purple or flesh colors.

By producing some chemical change it is often possible to alter the color of a mineral. In the case of minerals colored by hydrocarbons, these changes may best be produced by heating. In this manner smoky quartz can be changed in color to yellow, yellow topaz to pink, and brown carnelian to red.

Amethyst, hyacinth, and golden beryl lose their color entirely if heated any length of time, and smoky quartz may also be made colorless by long continued heat. Some gems change in color on heating, but regain it again when cooled. Thus pyrope turns darker on heating, but returns to its normal color on cooling. Ruby becomes colorless, but on cooling changes through green to its original red.
Some colors of gems fade or change on exposure to light, a peculiarity which is of course considered detrimental to their value. In this manner the blue of turquois may change in time to green, and yellow topaz, chrysoprase, and rose quartz may lose their color entirely.

Some gems are of a different color by artificial as compared with day light. The beauty of some may thus be enhanced by artificial light and that of others weakened. The gem in which the most striking change is thus produced is chrysoberyl of the variety known as alexandrite. This is green by daylight, but red by artificial light. Most yellow stones appear nearly colorless by artificial light because the excess of yellow rays in the latter makes those from the stone almost invisible. For the same reason violet stones are likely to lose much of their color in artificial light. One of the points of superiority of the emerald is that it is able to retain its color in all lights. The color of the ruby is deep ened and made more brilliant by artificial light, and turquois of good color has its effect enhanced by the same.