A crystal lamp is a stone with a light inside or beneath it. Whether it looks good depends almost entirely on translucency — a stone that light cannot get through is a stone with a bulb behind it, which is not the same thing.
Which stones actually glow
Selenite is the standout, because gypsum is naturally translucent and its fibrous structure carries light along the grain — a selenite tower lit from below glows the whole way up rather than just at the base. Rose quartz and clear quartz work well. Banded agate slices are excellent because they are cut thin.
Opaque stones — obsidian, most jaspers, hematite — do not light. They can be lit from outside and look handsome, but that is a lamp next to a stone.
The bulb is half the object
Use a low-wattage bulb or an LED. Two reasons. First, the effect: a bright bulb blows out the colour and you lose the banding entirely, whereas a dim warm light shows the internal structure. Second, heat.
Heat matters more than people realise with selenite. Gypsum contains water in its crystal structure, and prolonged heat will gradually dehydrate the surface and dull it. An old incandescent bulb sitting inside a selenite tower for hours a day is a slow way to ruin it. LED runs cool and solves the problem outright.
Not a salt lamp
Himalayan salt lamps are a different product with a different problem: salt is hygroscopic, so they weep in humid rooms and need to be kept warm to stay dry. Selenite and quartz lamps do not weep — but selenite must be kept away from moisture for a different reason, because it dissolves. The two get shelved together and behave nothing alike.
Practical points
Check whether a lamp comes with its bulb and what fitting it takes; small unusual fittings are common and annoying to source later. Check the cable exit — a lamp whose cable comes out of the side sits badly against a wall. And keep any selenite lamp off a windowsill above a radiator, which is the single most damaging spot in most houses.