A dark, rusty-brown surface coating of iron oxide, manganese oxide, and clay often found on desert rocks is known as

desert varnish

Explanation:

Desert varnish

Also called the rock varnish,

It is the orange - yellowish to blackish coating which is found on the surface of rocks in the arid environments.

It is about a micrometer thick and have a nanometer - scale layering on the surface.

desert varnish is formed only over the surface of the physically stable rocks.

The composition of the varnish is from iron, and manganese oxides.