My favourite changes with my mood but what popped into my head was negative masking! Cut a hole using a die or a punch or even freehand (fold your paper and cut a half shape if you want something symmetrical) and then sponge inks in there going from lightest over the whole area to darker at the edges -
here's an example from my gallery (hole cut with a Nestability die, image stamped with the mask still in place).
LetteringMama - Glossy Accents or Crystal Effects are basically the same thing as Liquid Glass - just different brand names.