Garnet can show what's known as anomalous double refraction from internal stress, though it is not the same as true double refraction. This could be almost sny purple stone, sapphire, amethyst etc.
so at least you can do some simple tests: rotate it in front of an lcd screen and see if it has any color change, check for refractive index, find its specific gravity. There are a LOT of different gems this could be, so it is very hard to hazard a guess without any basic information.
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