Quartz is a hard mineral, a kind of stone. The sense here is the one inside electronic watches and clocks. A tiny piece of quartz can be cut so that, when a little electricity runs through it, it shakes at a very steady speed. The watch counts those shakes, and that is how a quartz watch keeps time so well — even a cheap one. Shop signs still say “quartz” for that reason: the crystal is doing the timing, not a spring. Though actually, quartz is also one of the most common minerals on Earth. A lot of sand is quartz, and it turns up in granite and in glass. When a learner’s dictionary points at watches and clocks, they mean that small, steady crystal inside. The word is ordinary in shops and in science class; it does not feel fancy.