Probably the oddest idea I know of is how an ancient piece of coloured glass can improve your health. The experts use long and overly complicated terms to suggest they know how things work, and the believers who don't yet have this the spiritual lexicon will only say 'I don't know how it works, but it does.' In this kind of case, I don't know what's worst, the people saying they know because they can make poorly supported arguments sound smart, or the sad cases who accept the bad arguments and cannot explain why they get a kick out of crystals.
It doesn't have to be old and natural, just fused glass with a beautiful colour works great on people who don't know better. Such as blue obsidian, commonly this is just light blue glass and is quite popular as a polished pebble, which means many people are buying a machine smoothed piece of glass because it's a lovely colour. Traditional shapes for such items include pyramids and spheres, that pseudo-experts claim help energises an environment, help with channelling and cosmic energy, etc. There is nothing to the science-like claims other than the placebo built up by a new age mystics and salespeople. Even if they refer to ancient cultures liking fancy looking pieces of old glass, such as the long historical use of jade in the Far-East or ancient cultures having rituals which included them. The ancient Jewish high priest breastplate which is said to of had twelve gems in it to represent the twelve tribes of Israel, so the new ager with an Israelite fetish might buy each of these stones to get closer to spirit.
Pointing out where other cultures use gemstones doesn't confirm that since they had them, they must have thought of them as you think of them now. Even if they did, how does that prove anything real unless it is something demonstrable? To an Eastern mystic, a crystal may be a helpful thing to focus on, but if it is the centre alone then what does it matter if you carry a pebble or a piece of wood or bone? If a symbol in mind is enough for people to have engaging experiences in meditation then what use is a stone that is only as useful as a mental tool, and maybe that is what it truly is, a psychological tool.
Often the creation of a false history takes place, in which the selected stories and facts of the past are expressed as if we have lost a great secret. For decoration and superstition a gem encrusted crown, in the 12th century Europe, may convey a sense of power, but can we find anything near the magical ideas of the modern new age movement? If we find enough examples that we can rewrite as evidence in a pseudo-history when is that somehow real? The fact that you need to change things to make it fly says a lot about the honesty of the new age spiritualist' search for knowledge.
In essence, the practices of meditation and devotion with stones is a throwback belief, grounded more in thought-reform rather than practicality. The practice offers self-induced states of delusion, not real mindstates that would be far more attractive to critics. It's just collecting pebbles and carvings with the hope that they have magical properties.
Maybe the best thing you can say about the properties of crystals and minerals is that the spiritual/psychic claims cannot be utterly refuted. It's not unlike fairies and gods, they cannot be totally debunked in the minds of the faithful, but as science continues, there are fewer places for them to hide. One day science may shine some light on such topics, even if modern psychology and neurology explain the experience basis for spirituality.