Is there a difference between Density and Specific Gravity?

My textbook says that the specific gravity is "the ratio of the mass of a mineral compared with the mass of an equal volume of water."

Is this any different to density? If so, how? If not, why does it have its own name in geology?

• As mentioned in the other answers, SG is a ratio of densities. Turns out that density in g/cm³ has the same numerical value as SG. g/cm³ is used very commonly in geology, therefore these two are used interchangeably. So they are not exactly the same thing, but the number is the same and they mean generally the same. – Gimelist May 22 '16 at 10:25
• Using the metric system, SG & density have the same numerical value because the density of water is 1 g/cm³. In antiquated systems of units like the British or US system of units, numerically there will be differences between SG & density. – Fred May 23 '16 at 14:54

If you read your definition carefully you will realise that specific gravity is defined as the ratio of the density of the material to that of water. $$\mathrm{SG = \frac{\rho_{sample}}{\rho_{water}}}$$