ASA Digital Art’s observation, not an established finding: in our 1200x microscope photographs, part of the black surface on one of our specimens appears to become translucent when strong light strikes it from opposite directions. We wondered whether it might be a desert-varnish-like coating. The specimen is not identified as carbonado.
Desert varnish is real and well studied. The U.S. National Park Service describes it as a family of thin rock coatings; rock varnish proper consists mostly of clay minerals with substantial iron and manganese oxides, and higher manganese makes it blacker[1]. It forms on stable rock surfaces of many kinds, including sandstone, limestone, basalt and granite, and is most conspicuous in arid climates[1,2]. Its constituents derive from water flowing over the rock, windblown dust and microorganisms[1].
Carbonado’s darkness has a different published explanation. Haggerty attributes its opacity to randomly oriented crystallites that scatter and absorb light[3], and Trueb and De Wys described its texture in 1969[4]. Its surface often carries a glassy diamond patina only micrometers thick[3].
A coating that turns translucent under bright light could be many things, and a photograph alone cannot distinguish them. Testing would require analysis of the coating’s chemistry, for example for manganese and iron oxides, by a qualified laboratory.