Here's the thing… Just because something tests as a natural gem, doesn't mean it's worth anything. The difference between a flawless ruby and a very slightly included ruby is thousands of dollars per carat. It is all about quality. A gemstone is a specific specimen of that mineral that exhibits find characteristics of color, cut, and clarity.
Not saying that this is or isn't a natural garnet; I'm saying it doesn't matter because it doesn't have any value beyond a neat specimen to put on your shelf.