There are several variety of prism holders ranging from plain round rings to very ornate and decorative types that fit over the lamps collar. Some shade holders are also drilled around 
their edge to hang prisms from and are shown here as well.
The over the collar types vary in size to fit the lamp’s collar and to overhang the side of lamp for prisms to hang. Also the amount of prisms they hold will vary. Solar type lamps typically use the ring or shade ring type, although some early transitional glass kerosene fonts have an embossed lip around the font that the ring can sit onto. Girandoles and some candlesticks can also be found with prism holders.
Prism holders are more commonly found on higher end lamps, but add a special touch to any lamp.