Why hollow beads? From the perspective of jewelry designers, less is more.
Hollow beads, by definition, weigh less than solid beads of the same size, making them ideal design elements for necklaces and earrings. From the glass artist's perspective, there's something challenging and magical about making hollow beads.
Blown hollows are typically made with borosilicate glass tubing, and are made in much the same way as holiday ornaments. Inside-out beads are one example of this technique: The color is applied inside the tubing as the bead is made. Wound hollows are commonly made using "soft" glass. Two glass "walls" are constructed on a mandrel, then sealed together, trapping air between them. When heated, the trapped air expands and pushes out the softened glass walls, forming a puffy, lightweight bead.
(Thanks to Tink Martin for today's GlassFact. Tink is a master of hollow beads and small blown vessels. She teaches flameworking at the Toledo Museum of Art and is a frequent contributor to Wet Canvas.)