As the wife of a geology professor, I have seen my share of rocks and minerals. There's malachite on my desk, kyanite in the bathroom, and a wedge-shaped specimen of rock gypsum on the buffet. These are my personal favorites. To my unscientific eye, these museum-quality rocks and minerals get to come inside because they look, um...pretty.
Then there's obsidian. Though it's not as colorful as the garnets, fluorites or sulfur, its transformation from magma to glass is fascinating. You see, obsidian is formed when lava cools super-quick on the earth's surface. It's glossy black and sharp-edged, which made it ideal to fashion into arrowheads and other weapons by ancient people. Pliny the Elder referred to obsidian in his Natural History, and said it was named for its discoverer, Obsius, who first found it in Ethiopia.
I have a few chunks of obsidian in a rock border outside. When it rains, the water sits on the non-porous surface and evaporates quickly when the sun comes out. It reminds me of licorice.