Specific Gravity

Diamonds like every other mineral have a Specific Gravity, Specific Gravity (in the Diamond industry) is the ratio of the Diamond weight to the weight of an equal volume of water, in other words the ratio of the density of a Diamond to the density of water.
The rule is that higher the Specific Gravity the heavier a mineral is from an equal volume of a different mineral with a lower Specific Gravity.
 For examplae:
The Specific Gravity of a Diamond is 3.52, the Specific Gravity of a Cubic Zirconia is 5.80 (approximately), so a Cubic Zirconia of the same size as a natural Diamond will weigh more.