Minerals aren't rocks, but they often sit on rocks. As an example, quartz, feldspar, and mica are minerals. But, mix them together and you have granite, a rock. Basically minerals are one thing, while rocks are several of those things mixed together.
So your wifes diamond - rock or mineral?
Chilly
PS- it was a trick question. Another part if the definition of a mineral is that it is naturally occuring. A cut diamond is a gemstone.