A 7 gram rock is quite small in size, roughly comparable to the size of a small pebble or a marble. The size of a rock weighing 7 grams depends on its density, but for typical stones with densities around 2.3 to 2.9 g/cm³, a 7 gram rock would have a volume of about 2.4 to 3 cm³. This translates roughly to a rock about 1 to 1.5 cm (around half an inch) in diameter, roughly the size of a small grape or marble.