Joined: Sun Oct 16, 2005 12:22 pm Posts: 21602 Location: San Francisco
It is quartz. No fossils; it contains inclusions of other minerals.
The Quartz Page wrote:
Rainbow or Iris Quartz There are two types of quartz crystals that are called rainbow or iris quartz.
One is fairly common: Quartz crystals often show cracks running through them, usually caused by mechanical stress, and these sometimes show rainbow-colored reflections. Due to the conchoidal fracture of quartz the cracks are curved and the width of the fissure is not the same over the whole extent, but changes gradually. The cracks are usually very thin, and often just filled with air. Light that passes through the crack will get partially reflected on both walls of the crack. If the width of the crack is of the same order of magnitude as the wavelength of light, both reflected light beams will show interference colors: Parts of the spectrum will be canceled out, and the reflected light appears colorful. The color depends on the width of the crack and the angle at which the light passes the crack. As the width of the crack changes gradually, so does the color and this causes a rainbow-colored reflection. This effect is known as iridescence (hence the name iris quartz) and can also be observed when thin layers of oil swim on water.
Normally you can't identify stones 100% from a photo, but this is one of those rare exceptions. The color and crystal form are extremely distinctive. The only thing it could be other than quartz is an extremely sophisticated fake meant to imitate quartz, which doesn't seem likely.
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