A good rule of thumb is to use the largest wire that will fit through your bead channels once. This will ensure that you are using the strongest, most durable wire.
If you are mixing and matching your heavy stones with smaller spacers and beads, using a .019 diameter will probably be the easiest to work with if the smaller beads have tiny channels. If your design is just going to be large, chunky stone - .024 would be ideal.
Stone, like Turquoise, can be abrasive and rough on the wire, .024 is the wire we specifically manufacture for this kind of abuse. .024 has a test strength of 40 pounds, that is, it takes 40 pounds of snapping to break it. It is a very sturdy wire. We carry a line called Soft Touch if you are concerned about drape.
Happy Beading!