Hey Sid! I know its hard to tell because there are no images or location of your mineral for me to go off of, but before I take a guess, a helpful thing you could potentially do is do the Mohs hardness test to determine what category your mineral could be in. However, If I had to take a guess, I would guess the rock you have to maybe be in the Igneous family. Again, as an educated guess, maybe you have what is called Gabbro, an igneous rock formed by magma, is dark grey to black color and can have a shiny surface caused by the feldspar inside of it if it's chipped off or can naturally be shown. Hope that can somewhat help you in finding out what it is! Have a good day.
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