Some nouns like "apple" and "watermelon" are sometimes count nouns, and sometimes non-count nouns, depending on how we're thinking about them, not whether they're solid or broken.
When we're thinking of individual pieces of fruit, "apple" and "watermelon" are count nouns:
I have an apple in my lunch.
We almost ate a whole watermelon.
When we think of them as ingredients or a general substance, they are non-count nouns:
This pie needs more apple.
There's watermelon all over the lawn.
With "mirror", generally speaking, when it's broken, we don't think of it as a mirror anymore because it doesn't reflect images, so it's not a mirror. We think about it as a general substance:
"Our living room mirror broke, and now there's glass all over the carpet."
Mirrors stop being mirrors when they break, and become glass.