"Thin slab of Brazilian agate" by John I. Koivula

Submitted to the Nikon photomicrography competition in 1990, this entry won 4th place. In addition to just being a lovely piece, it has the distinctive signs of nature repeating herself - this time in the form of an alien desert landscape! And THAT, is a tell-tale sign that some natural process art is afoot!

https://www.nikonsmallworld.com/galleries/1990-photomicrography-competition/thin-slab-of-brazilian-agate