Prefabricated all-masonry fireplaces are one of the latest developments in man's long quest for a cozy fire in the home. For contractors, they make building a masonry fireplace a lot easier. Because the precast units are custom-shaped, more complicated, heat-efficient fireboxes and flue systems are possible. A FIREPLACE WITH MULTIPLE FLUES One prefab fireplace on the market features a flue that, as it leaves the firebox, divides into six or eight smaller flues. Each small flue zigzags back and forth for several feet, then reunites with the other small flues into one large flue. The zigzags and increased number of flues give the flue system 3 1/2 times the surface area of a normal fireplace flue. SOAPSTONE PREFAB FIREPLACES Several models are available, but they all work on the same principles: a contraflow flue and the extraordinary heat-absorbing properties of soapstone. The labyrinthine flue allows most of the soapstone in the fireplace to be exposed to heat, which soapstone readily absorbs and releases into the room over several hours. PREFAB FIREPLACES MADE WITH VOLCANIC PUMICE Prefabricated fireplaces made from pumice-based precast units were introduced in the United States in 1989. The precast units are made from volcanic pumice aggregate and portland cement. The material is so fire-resistant that the fireplace can be built with zero clearance from combustibles.