Read the passages carefully and answer the questions that follow:
Saddle querns … are found in considerable numbers …. And they seem to have been the only means in use for grinding cereals. As a rule, they were roughly made of hard, gritty, igneous rock or sandstone and mostly show signs of hard usage. As their bases are usually convex, they must have been set in the earth or in mud to prevent their rocking. Two main types have been found: those on which another smaller stone was pushed or rolled to and fro, and others with which or rolled to and fro, and others with which a second stone was used as a pounder, eventually making a large cavity in the neither stone. Querns of the former type were probably used solely for grain, the second type possibly only for pounding herbs and spices for making curries. In fact, stones of this latter type are dubbed "curry stones" by our workmen and our cook asked for the loan of one from the museum for use in the kitchen.
What materials were these querns made of?

Explanation

The querns were made of hard, gritty, igneous rock or sandstone.