General Identification of Gems
Gem identification is basically a process of elimination. Gemstones of similar color undergo non-destructive optical testing until there is only one possible identity. Any single test is indicative, only. For example, the specific gravity of ruby is 4.00, glass is 3.15-4.20, and cubic zirconia is 5.6-5.9. So, one can easily tell the difference between cubic zirconia and the other two; however, there is overlap between ruby and glass.
And, as with all naturally occurring material(s), no two gems are identical. The geological environment they are created in influences the overall process so that although the basics can be identified the presence of chemical "impurities" and substitutions along with structural imperfections vary thus creating "individuals".
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