Certain crystals of fluorspar (fluor or fluorite), especially the twinned cubes from the lead mines of Weardale in County Durham, England, display different colors when viewed from different points of view. The color as seen varies according to the relative positions of the source of light, the specimen, and the eye of the observer. When the crystal is between the source of light and the eye one color is seen, and when the eye is between the crystal and the source of light another color appears. The difference depends on whether the crystal is viewed by transmitted or by reflected (scattered) light. Holding a clear transparent crystal up to the window, the color seen may be pale shades of brown, pink, green (in some crystals a good emerald-green), or colorless; these colors being usually arranged in alternating layers parallel to the cube faces of the crystal. But when the same crystal is viewed by reflected light, with the back of the observer towards the window, a totally different color is seen. This appears, especially in direct sunlight, as a rich bluish-violet glow or shimmer diffused over the surface of the crystal, very much like the bloom on a ripe plum. When a lens is held in front of the crystal, a conical beam of sunlight inside the crystal is traced out by this color.

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