Ceramic is very hard. Not in the general, colloquial sense of hard, but in the Mohs Hardness Scale. That scale measures whether or not something can scratch something else. Diamond is the hardest on this scale - not because you can't break diamond. Diamonds are pretty brittle, you can easily crack or shatter them with a hammer. But you can't scratch them.

Tempered glass is made by putting the glass in tension. You cool the glass so that the outside shrinks, compressing the still-hot inside. When the inside then cools, the outside is frozen in place so the inside pulls tightly on it the inside contracts. The glass pulling on itself keeps the molecules from moving, and holds it all together strongly so it's very hard to break.

By scratching the surface, ceramic breaks the tension, releasing all of it throughout the glass.