What does 18k thg mean inside a ring?

Oh, dude, 18k THG inside a ring stands for 18 karat gold, which means the ring is made of 75% pure gold mixed with other metals. THG is just the manufacturer's stamp or trademark, so it's not some secret code or anything. It's like the ring's way of saying, "Hey, I'm fancy and valuable, but not too fancy to hang out on your finger."