This article is about the large rocks. For other uses, please see boulder (disambiguation).

In geology, a boulder is a rock with grain size of usually no less than 256 mm (10 inches) diameter. While a boulder may be small enough to move or roll manually, others are extremely massive. Some famous rock formations involve giant boulders exposed by erosion, such as the Devil's Marbles in Australia's Northern Territory.

In common usage, a boulder is too large for a person to move. Smaller boulders are usually just called rocks or stones.

Bouldering is a sport that involves climbing on boulders.

Boulder, Colorado is also a city in the United States

Geology | 岩石