The perfect bead for tying midges like the Pearl Jam, Periwinkle and any other small flies that require a small bead. Pat Dorsey's Mercury series are tied with an x-small clear glass bead. This includes the mercury baetis, mercury pheasant tail, mercury black beauty, mercury midge, and the mercury blood midge among many others. The other colors are great for creating small midges and nymphs - get creative!
Hook sizes for x-small glass beads: #20-#24
So far the beads have met my expectations. But I have only had a chance to tie 6 flys with them.