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On the outside, this rock looks like a rough white egg. But inside it looks like Cookie Monster, the Sesame Street Muppet! 

This special rock is a type of crystal called agate. Agate is known for its colorful stripes. It’s formed from a mixture of volcanic rock and tiny crystals called quartz. 

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