Plastic Watch Crystal = Fake Rolex?

A simple question from a total watch noob. I was going through some old stuff in my parents basement and stumbled across a few old watches. I found a rolex, and my mother was the kind of person who'd own a real Rolex(malpractice defense attorney). The problem is that I noticed the lens on it is cracked and made of plastic. Seems too cheap of a feature to be on a Rolex, but also it seems it would be too much of a giveaway on a fake since the rest of the watch looks very convincing. Did some