p Nevada wonder stone. This volcanic rock is a rhyolitic air-fall tuff, material ejected from a volcano about 12 million years ago. The rock was altered by hot waters that deposited pyrite (FeS₂) and quartz (SiO₂). Rainwater penetrated the rock and oxidized the pyrite to form liesegang bands of red hematite (Fe₂O₃) and orange and brown goethite (FeO(OH)). Erosion broke pieces of the rock from its outcrop, and streams carried pebbles to the shore of Lake Lahontan, which covered much of western Nevada during the Pleistocene Epoch. These pebbles, found along the road two miles east of Grimes Point, were tumbled and polished by the streams and by wave action along the shore of the lake about 13,000 years ago.

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