This kind of preservation is largely found in dinosaur fossils within the Jurassic Morrison Formation, a sedimentary rock formation found in Wyoming, Utah and Colorado. [Image via http://imgur.com/gallery/2OFmi]
This image depicts bands in a marine sedimentary agate containing blebs of iron oxides at 32 times magnification. It was awarded image of distinction in the 2012 Nikon small world photomicrography competition.
Holly Blue Agate was pretty much a Mary Sue gemsona but canon. Like… just imagine:
“My gemsona is Holly Blue Agate but she goes by Holly. Her gem is on the back of her head and her hair looks like cool striped cat ears. She’s big and powerful and works directly with Blue Diamond to capture and preserve humans in this cool human zoo thing. Also her weapon is an electric whip and she hates everybody because she’s better than all of them but everyone admires her and looks up to her RAWR XD”