Here's why:

* Hardness: Agate is relatively hard, making it durable for grinding and crushing.

* Inertness: Agate is chemically inert, so it won't react with most substances you might grind in it.

* Smoothness: The surface of agate is smooth, which helps prevent material from sticking to the pestle and mortar.

While quartz itself is hard, it's not as durable or smooth as agate for these purposes.

It's important to note that not all pestles and mortars are made from agate. Some are made from other materials like porcelain, ceramic, or even wood. However, agate is a very popular choice due to its properties.