Glass pearls are glass beads on which a opalescent outer shell has been deposited. The advantage of glass pearls is that, unlike other pearls, the original diameter hole is maintained so stronger or larger stringing material can be used with the pearl. Pearls usually have the smallest hole in the world of beads, which makes them hard to work with and not likely candidates for beaded bracelets, etc. The larger holes in glass pearls allows them to be used in the same manner as any glass bead.
At this time, the best glass pearls are made by the Swarovsky Corporation, so you know the quality will be of the finest. They use a crystal bead as the core and cover it in several layers of their proprietary pearl coating. The lead crystal core makes the bead heavier than other pearls. With the control the company has on the coating material, the Swarovsky crystal pearls are consistent in color and have flawless surfaces. The Swarovsky glass pearls come in many colors, some quite strong, and in several sizes.
Other bead manufacturers also make pearls by coating glass beads with layers of pearl-like material. Since their quality control may be a bit more lax, the pearls may vary slightly in size, color and/or retaining the absolute shape of the original bead, which itself may vary some. The variation is not a bad thing, for most projects are enhanced by the minor variations in the beads. With the spread of glass beads/opalescent coating technology, pearls have become very reasonable in price and one can have colored pearl necklaces and bracelets to go with every outfit is desired.
So, consider collecting glass pearls, especially the Swarovsky ones. The quality of Swarovsky manufacture means they will hold their value better than most pearls, except the pearls that have been in the family for generations.