Calliostoma fragum has what might be considered an unusual diet. It is a fire coral eater. In the Marshalls it appears to eat exclusively fire coral of the genus Millepora.
This snail was not especially rare, but could be found living only at night and only on colonies of live Millepora. A Millepora colony often covers several square meters and can support up to ten or so of various sizes of these small predators, although most colonies of the coral are free of the little parasites.
The shells from the Marshall-Islands seem to max out at about 9mm tall.
According WoRMS: This top snail is an algae grazer and deposit feeder.