<P> Gems are characterized in terms of refractive index, dispersion, specific gravity, hardness, cleavage, fracture, and luster . They may exhibit pleochroism or double refraction . They may have luminescence and a distinctive absorption spectrum . </P> <P> Material or flaws within a stone may be present as inclusions . </P> <P> Gemstones may also be classified in terms of their "water". This is a recognized grading of the gem's luster, transparency, or "brilliance". Very transparent gems are considered "first water", while "second" or "third water" gems are those of a lesser transparency . </P> <P> There is no universally accepted grading system for gemstones . Diamonds are graded using a system developed by the Gemological Institute of America (GIA) in the early 1950s . Historically, all gemstones were graded using the naked eye . The GIA system included a major innovation: the introduction of 10x magnification as the standard for grading clarity . Other gemstones are still graded using the naked eye (assuming 20 / 20 vision). </P>

State one physical property of garnets other than color which make them useful as gemstones