Cookie Crystal

On the outside, this agate rock looks like a rough white egg. But crack it open, and it looks like Cookie Monster, the Sesame Street Muppet! Agates are a type of quartz—crystals of the chemical silicon dioxide—known for their bands of color. They form when hot water rich in silicon dioxide enters holes within volcanic rock and produces microscopic quartz crystals. The color usually comes from impurities like iron trapped in the crystals.

“A lot of agates look like they have a smiling face because they have a void in the middle. Then crystals grow and it kind of looks like teeth,” says Shannon Mahan, a geologist with the United States Geological Survey. “But I’ve never seen anything so Cookie Monster-like.”