I have some geology in my background. I would define a gemstone as an entirely inorganic, natural crystal. Lab-grown crystals with the same chemical composition and structure would also qualify since they are indistinguishable from the real thing. Substitutes like cubic zirconia would not. Glass is not usually natural, and sea glass, although beautiful and aesthetically pleasing, is technically trash. I have a hard time reconciling it with the word gem, just as I wouldn't call marble a gem. Marble is a rock. There's just more to the word "gem" for me hard and pretty.