Conversation so far:
who is burried in the tomb of the unknown soldier
The Tombs of the Unknown Soldiers typically contain the remains of a dead soldier who is unidentified as these remains are considered impossible to identify.
Who is buried in the Tomb of the Unknown Soldier in Arlington, Virginia?
On March 4, 1921, the United States Congress approved the burial of an unidentified American serviceman from World War I in the plaza of the new Memorial Amphitheater.

Question: what material is the tomb made of
Self-contained rewrite: Where does the building material for the Tomb of the Unknown Soldier come from?