Abstract: An interlocking construction block of thermally insulative material which is of a decorative nature and is capable of being erected without mortar. The block, which may be of construction block size, or of the dimensions of conventional brick, is molded of a synthetic insulative material and includes faces into which real brick inserts may be molded. The block upper wall includes a pair of truncated conical projections adapted to closely nest within conical recesses formed in the lower surfaces of the block above, and the blocks are provided with openings and finger grasping ledges to facilitate handling, and the end walls include elongated grooves for receiving interlocking pins. Fastener receiving recesses are defined between the projections, and the block includes voids at noncritical areas to reduce the mass. A foundation slab for use with the block of the invention is disclosed, as are caps when building walls.