If I make a necklace that has the gems floating, what type wire is good and what type of crimp bead is good for keeping the bead in a suspended hold?
The best crimp to use is one with a smaller profile yet large enough in diameter to accommodate the wire or cording you choose, and be large enough to not go through the hole of the bead you want to suspend.
If using 0.019-inch or 0.024-inch diameter Accu-Flex® beading wire, 0.025-inch diameter Invisible cording or 8-pound test, 0.018-inch diameter Spiderwire®, a micro crimp or one that falls in the range of 1 - 1.5mm in diameter would work well. If you choose a heavier diameter cording or wire, be sure to increase the size of the crimp accordingly. Capture the wire with the crimp by either compressing it with a pair of chain-nose pliers or crimping pliers depending on the look you want to achieve.
You could also use an adhesive to keep the beads in place, instead of a crimp. Loctite® 454 Gel adhesive is the ideal adhesive for this as it works well on smooth surfaces and when you need instant bonding.
Bead Huggers™ are yet another alternative when keeping beads in place. Depending on the diameter of the wire you choose, you may find that gluing these in place will keep them and the "floating" beads where you want them.