Imagine my surprise when I put a rock into my tumbler and it came out looking as unusual as this.  It is made of a fine sedimentary rock, like siltstone or compacted ash layered with what looks like some type of grayish brown to black common opal or silica-rich lava flow, similar to an obsidian.    No matter how it was formed, it has turned out to be an almost industrial looking piece of natural art that can be displayed on a shelf, used as a paperweight, or in a desert scene as a crafting material.