Opal
One of the most prized of gemstones, opal has a reputation for being highly unlucky. This is perhaps due to the nature of the stone itself. Opal is not a true crystal but rather a suspension of microscopic spheres held in a jelly-like solution. Opal contains 3% to 30% water, and herein lies the source of its misfortune. The stone eventually dries out and begins to lose its luster. Repolishing stresses the stone even further (it has a hardness of 5½ to 6½ on the Mohs' scale). To add insult to injury, opal will not tolerate extremes of temperature, and the slightest provocation may cause the stone to break.
Despite all these drawbacks, opal has been a highly valued stone since ancient times and has been known by a variety of names. Nearly every medieval lapidary mentions the stone, although one cannot be certain that the opal of ancient times is of the same species as the modern stone. Other gem stones, such as cat's-eye, tiger's-eye, and moonstone also exhibit iridescence and play-of-color.
Albertus Magnus thought this play of colors to be bad for the eyesight and associated opal with diseases of the eye. His description of panthera seems to describes "black opal", a stone otherwise thought to be unknown in medieval times. Possibly he had some other stone in mind, such as Finnish spectrolite.
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