peradam

"One finds here, very rarely in the low-lying areas, more frequently as one goes farther up, a clear and extremely hard stone that is spherical and varies in size--a kind of crystal, but a curved crystal, something extraordinary and unknown on the rest of the planet. 

Among the French of Port-des-Singes, it is called peradam.  The clarity of this stone is so great and its index of refraction so close to that of air that, despite the crystal's great density, the unaccustomed eye hardly perceives it.  But to anyone who seeks it with sincere desire and true need, it reveals itself by its sudden sparkle."

from Mount Analogue: A Novel of Symbolically Authentic Non-Euclidean Adventures in Mountain Climbing by René Daumal

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