This depends on the size of gravel, the water content of the gravel, and the material composition of the rock used to make the gravel. Typically 1 cu.yd. of gravel wieghs 3000 lb.
According to the CRC Handbook of Chemistry and Physics (60th Edition), a cubic foot of clay weighs 112 to 162 pounds. A cubic yard would weigh 27 times as much.
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