Question

find the appositive. if it is nonrestrictive, set it off with commas. if it is restrictive, submit the text without making any changes.
the stone of scone a block of sandstone has been used in coronation ceremonies in britain for hundreds of years.

Explanation:

The phrase "a block of sandstone" is a nonrestrictive appositive: it provides extra, non-essential descriptive information about "The Stone of Scone" (the stone's identity is already clear without this detail). Nonrestrictive appositives require commas to set them off from the main clause.

Answer:

The Stone of Scone, a block of sandstone, has been used in coronation ceremonies in Britain for hundreds of years.