Optical Properties of Minerals
The specimens on the left show some of the glorious variety of color in minerals. Color in minerals results from the way light interacts with certain atoms, or groups of atoms, present in the mineral.
It can come about in three ways. First, the color-active atoms may be a principal component of the mineral, such as copper in the dioptase specimen, number 10. Second, the color may be due to trace amounts of an atom or a mistake in the crystal structure. For example, iron in quartz produces amethyst, specimen number 3. Finally, the color effect may be produced by the inclusion of small particles of a different mineral of substance in the mineral, as with hematite inclusions in quartz, resulting in jasper, specimen number 19.
The yellow of sulfur and the golden metallic sheen of pyrite are examples of color that can be used as a diagnostic property. In many minerals, however, the color can vary over an immense range, mostly because of impurities. Even in such instances, the color range of a mineral can be a useful diagnostic property.
In the center of this case, some special optical properties are displayed. These properties occur because of interference in the light rays as they strike and interact with the mineral, causing a play of colors called iridescence. The labradorite, specimen number 41 in the lower part of the case and the opal, in the middle, are particularly striking examples of this property.
The reflection of light in a starlike pattern, from the polished piece of corundum, specimen number 38, is known as asterism, and hence this specimen is called a star sapphire. This effect is well displayed in the Star of India in the Morgan Gem Hall.
Displays at the right present some very important optical properties. First is the phenomenon known as streak, which is the color of the powder made from a mineral. Intriguingly, the color of the powder does not always match the color of a mineral. This contrast is well-demonstrated by the white powder and the lavender crystal of specimen number 46, flourite, at the upper right.
The next group of minerals illustrates various forms of the property known as luster, the surface appearance of a mineral. Moving from the resinous sulfur to the silky asbestos, to the waxy chalcedony, it is easy to see that the mineral kingdom displays a wide range of lusters.
Bear in mind that these different lusters are caused not only by the kind of mineral, but also by the mineral's texture and by the way the mineral grew. The last group of minerals portrays another property important in identifying minerals.
Light of all wavelengths has a constant speed in a vacuum. But when it moves through other media, such as glass, or water, or a mineral, its speed depends on the density of atoms in the substance it meets and, therefore, varies markedly among different mineral specimens. This property, known as refractive index, is illustrated by specimen number 62 through 72.
The last group of minerals in this case exhibits other special properties, such as single refraction, double refraction, polarization, pleochroism and dispersion.
Streak is the color of powder made from a mineral.
Mineral surfaces may show one of two major types of luster: metallic and nonmetallic.
In general, when light passes from one medium such as air to another such as a transparent mineral, it is bent or refracted.
Light entering minerals with low crystal symmetry is broken into two rays.