Optical Properties of Minerals main content.

Optical Properties of Minerals

Part of Harry Frank Guggenheim Hall of Minerals.


Exhibition Text

Audio Transcript
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.

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