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Gem materials vary greatly in density -- amber may float in salt water (density near that of water), while hematite is more than five times the density of water. This is why two different gemstones may have the same size but different weights and vice versa -- a one carat round brilliant diamond of typical proportions will be approximately 6.5 mm in diameter, while a round brilliant ruby of the same size (6.5 mm in diameter) and proportions will weigh approximately 1.55 carats. Generally, gemologists refer to specific gravity, or relative density -- the ratio of the density of a gemstone relative to that of water.
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