Most gemlore was made up in the last fifty years or so – as a way of taking money off gullible new agers.
Traditional gemmology is hardly ever about healing (gems have other meanings and functions in the true hermetic tradition). But new agers aren’t interested in actually learning stuff, they just like wearing bling.
We don’t. It’s all psychosomatic. Somebody made it up (within the last few decades in most cases) and if you believe it, it might help – or may make you think it helps. There’s a whole placebo effect, confirmation bias thing going on with “gemstone healing” – and a lot of money to be made off gullible and/or desperate people.
In Christianity, we do believe that rocks do expel energy because Jesus says in Luke 19:40 that if God’s people stop praising, then the rocks will begin to cry out. If those are just the rocks praising him, then what greater healing power would God himself, who we believe created the rocks possess?