Magic Lantern
Summons a magic lantern that exposes nearby treasure
An enchanted lantern is lighting your way
2a Spelunker Potion you never have to brew
The lantern follows you around and sparkles anything valuable in a short radius. That radius is much smaller than the potion covers, so it will not map out a cavern for you, but it never runs out. Over a long mining session it saves an enormous number of potions and lights your way at the same time.
It sits slightly ahead of and above you, phasing through walls to keep up. When it parks itself over something you were trying to look at, turning around moves it to your other side.
getting the Skeleton Merchant to sell it
He only stocks it on a full moon night, which is the real obstacle. Two approaches work. Trap him near a cavern base, keep a Sextant on hand to read the moon phase, and sleep through nights until the right one comes up. Or make a throwaway world, dig to the cavern layer and hunt him down on the first night, which is always full. If he has not turned up by dawn, roll another world and try again.