In the great mosque (1.9.7) there are red and white stone wedges that make up the series of repeating arches that dominate the interior. what are these stone wedges called?

These stone wedges are called voussoirs.

Voussoirs are wedges, a type of stone often used for making arches and vaults - in this case, voussoirs were used to create arches in the Great Mosque (possibly referring to the Greate Mosque of Mecca, in Saudi Arabia).