Any of you who have strolled through the desert looking at rocks have prob'ly noticed that there are a great variety of weathering rinds and desert varnishes which look more or less like fusion crusts, depending on the type of rock and location in the desert. Since fusion crusts are glassy, they are isotropic and appear black under crossed polarized light. Are any of the desert faux-crusts also isotropic? Is this a reasonable way to tell 'rite from 'wrong? If you found a basaltic rock with a thin black rind in the desert and the rind looked translucent red and isotropic in thin section, would you say "I got a meteorite" or "My, what an interesting caliche"? What other ways of differentiating rind from crust are there?
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