Properties Depending Upon
Light and Color of Minerals

Color of Minerals
The color of minerals is one of their most important physical properties. In the case of many minerals, especially those showing a metallic luster, color is a definite and constant property and will serve as an important means of identification. For example, the brass-yellow color of chalcopyrite, the blue-gray of galena, the black of magnetite, the green of malachite, etc., is in each case a striking property of the mineral. It is to be note, however, that surface alterations may change the color even in minerals whose color is otherwise constant. This is shown in the yellow tarnish frequently observed on pyrite and marcasite, the purple tarnish on bornite, etc. In noting the color of a mineral, therefore, a fresh surface should be examined. Many minerals, however, do not show a constant color in their different specimens. This variation in color in the same species may be due to different causes. A change in color is often produced by a change in composition. The progressive isomorphous replacement of zinc by iron in sphalerite will change its color from white through yellow and brown to black. The minerals of the Amphibole Group show a similar variation in color. The amphibole tremolite, which is a silicate with only calcium and magnesium as bases, is very light in color, at times almost white; while actinolite and hornblende, which are amphiboles that contain increasing amounts of iron, range in color from green to black. Again, a mineral may show a wide rage of color without any apparent change in composition.  Fluorite is a striking example of this, since it is found in crystals that are colorless, white, pink, yellow, blue, green, etc. Such extreme vases are, however, rare. Minerals are also frequently colored by various impurities. The red variety of quartz, know as jasper, is colored by small amounts of hematite. From the above it is seen that, white the color of a mineral is one of its important physical properties, it is not always constant, and must therefore often be used with some caution in the identification of a species.

Play of Colors
Iridescence, Opalescence, etc. A mineral is said to show a play of colors when on turning it several prismatic colors are seen in rapid succession. This is to be seen especially in the diamond and precious opal. A mineral is said to show a change of color when on turning it the colors change slowly, being different for varying positions. This is observed in labradorite.  A mineral is iridescent when it shows series of prismatic colors in the interior of the crystal or on the surface. It is usually caused bye the presence of small fractures or cleavage planes which serve to break up the light into the prismatic colors. Opalescence is a milky or pearly reflection from the interior of a specimen. It is observed at times in opal and cat’s-eye. A mineral is said to show a tarnish when the color of the surface differs from that of the interior.

Asterism. Some crystals, especially those of the Hexagonal System, when viewed in the direction of the vertical axis, present starlike rays of light. This arises from peculiarities of texture along the axial directions, or from some inclusions. A remarkable example is the star sapphire.

Phosphorescence. Several minerals when rubbed or heated give out light. This property is known as phosphorescence. Fluorite often shows phosphorescence when fragments are gently heated. The color of the emitted light may be green, purple, rose, yellow, etc.