Bismuth crystals are said to have a "hoppered" shape. They form when molten bismuth solidifies, and get their rainbow sheen from an oxide layer that varies in thickness on their surface. These crystals don't tend to form in nature, and uncrystallized bismuth appears as a silver-white metal. Bismuth compounds are used in cosmetics and some drugs, most notably Pepto-Bismol.
Bismuth Crystals Look Like Spiraling Rainbow Staircases
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Key Facts In This Video
Bismuth has a low melting point for a metal: 271 degrees Celsius (520 degrees Fahrenheit). 00:18
A layer of oxidation gives bismuth crystals their rainbow colors. 01:27
See a man-made bismuth "geode": 02:28