Hardness measures the ability of a mineral to be scratched on the surface. A relative hardness scale (Moh's Hardness) is defined by 10 specific minerals, each of which can scratch anything below it (lower number) in the scale, but cannot scratch anything above it (higher number).

This scale is rated from 1 to 10, with talc as 1 and diamond as 10. The steps appear equivalent, but are actually not. The absolute hardness scale measures the true hardness of the minerals, and is depicted in the table below. As you can see, corundum is more than 5 times as hard as topaz, and diamond is in a class by itself.

Some simple equivalents are also listed to the right, and represent approximate" hardness values that can be used in the field to determine a rough hardness of a mineral.

Density is the weight of a material per unit volume. All this really means is that to compare one materials heft to another materials heft, we should use the same volume of material. We should take 1 square inch or 1 cubic centimeter and compare it with the same size piece of something else. Officially density units are gm/cm³.

Since it is not practical to cut everything up to exactly the same size, there is an alternative. We can measure the specific gravity. We measure the weight of the object, and then we measure how much water it displaces. The specific gravity is equal to the weight of the water displaced divided by the displacement. Since water is 1 gm/cc, the specific gravity happens to be numerically equal to the density.

Minerals containing elements with high atomic numbers are heavy and tend to be very dense. Gold has density of 19.3 gm/cc, and galena (containing lead (Pb) has a density of 7.5 gm/cc. Most of the minerals in the crust have densities between 2.5 - 3.0 gm/cc, so it is often possible to speculate that a mineral contains heavy elements when it has higher density.

These three factors are responsible for most of the physical properties of minerals. If you understand what each does, then you can predict (to a fair degree) what the properties will be for a mineral.

Conversely, if you measure the properties of a mineral, then you can give a pretty good guess as to the nature of the crystal structure, and the bonding. To a lesser degree you can guess the elements involved.