Pure minerals are rare - most rocks are a combination of many minerals.
The specific gravity of a rock or mineral is its weight, expressed as a multiple of the weight of an equal amount of water. Silicates, which are the most common minerals in the earth's crust, typically have specific gravities of 2.5 - 3.0. Metallic minerals are heavier, with specific gravities of 5 - 8. The hardness of a rock is its resistance to abrasion. This is a very useful way of identifying minerals.