Which term defines the ratio of the weight of a material to the weight of an equal volume of water?

• A. Growth Marks
• B. Singly Refractive (Isotropic)
• C. Specific Gravity
• D. Graining (Grain Lines)

Answer: (C)

Specific gravity is the ratio of the weight of a material to the weight of an equal volume of water. It’s a dimensionless measure because both weights are measured under the same conditions, and water serves as the reference with density defined as 1 g/cm3, so the value effectively reflects the material’s density in g/cm3. This property is useful in gemology for identifying and comparing materials, since different substances have characteristic SG ranges (for example, diamond around 3.52, quartz about 2.65). Other terms describe growth features or how light interacts with a crystal, not how heavy a material is for a given volume, so they don’t address this ratio.