Wikipedia
An altar stone is a piece of natural stone containing relics in a cavity and intended to serve as the essential part of an altar for the celebration of Mass in the Latin Church. Consecration by a bishop of the same rite was required. In the Byzantine Rite, the antimension, blessed and signed by the bishop, serves a similar function.
The Altar Stone is a central megalith at Stonehenge in England, dating to Stonehenge phase 3i, around 2600 BC. It is made of a purplish-green micaceous sandstone and is thought to have originated from outcrops of the Senni formation of the Old Red Sandstone in Wales, though this is currently in debate. Stone 80 (Altar Stone) was most recently excavated in the 1950s, but no written records of the excavation survive. Stone 55 (a sarsen megalith) lies on top of Stone 80 perpendicular, thought to have fallen across it. The Altar Stone weighs approximately six tons and would have stood nearly two metres tall. It is sometimes classed as a bluestone, because it does not have a local provenance. Now recumbent, it is thought to have originally stood as a single large monolith.
Its name probably comes from a comment by Inigo Jones who wrote: "...whether it might be an Altar or no I leave to the judgment of others’.
Usage examples of "altar stone".
My attendants tell me that the servants here say there is an altar stone there where terrible sacrifices were performed.
Inside the altar house she knelt beside the white altar stone—.
Lavastine surveyed the curving stone walls and the tiny carvings of snails and rosettes that adorned the altar stone.
The pillar was brightly painted red to the midpoint and green to the top, and it rose from a huge altar stone quite appropriately heavily stained.
When it was all done but for lighting the fires and triggering the circle of power with my blood, I sat upon the altar stone, opened my being to the cosmos, and prayed.
Tuvok feared that the altar stone itself would crumble under the continual assault of the earthquake before he was able to reach the level where Vok lay.
He struck full against the altar stone, banging his thighs, and held himself up against the cold stone.
Her father had gotten a length for the altar stone of their church and a little piece to hold a holy relic, and he had given her a piece, too, of which she had made her wedding gown.
They stood then, and Michael went forward to kiss the altar stone.
Blood pumped out of the open wound, washing the altar stone, darkening it farther.