desert varnish

noun
  1. the dark, lustrous coating or crust, usually of manganese and iron oxides, that forms on rocks, pebbles, etc., when exposed to weathering in the desert.

Origin of desert varnish

First recorded in 1900–05
Also called desert polish.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018