This wouldn't so much be glass, as sintered sand. Energy spent rendering the sand fully amorphous would be wasted. There are analogous igneous rocks, they aren

This wouldn't so much be glass, as sintered sand. Energy spent rendering the sand fully amorphous would be wasted. There are analogous igneous rocks, they aren't good for knapping into blades, leading me to think that broken glass trees wouldn't be especially dangerous either. Frits used in chemistry are sintered glass, and they don't break into sharp edges either.