Should you keep it?
Well, was it presented to you as a CZ without subterfuge? That is, he didn’t lie and tell you it’s a real diamond, right?
If he presented it to you honestly and truthfully, then that’s good. If he lied, then return the ring and dump him.
Now, assuming you have a truthful beau, I’d like to know if this ring was given as part of an engagement request? or just as an ordinary gift?
If part of an engagement, then you might want to consider your relationship and your betrothed moreso than the ring itself. If you intend to marry and perhaps raise a family, a significant consideration is whether your husband-to-be will be able to provide sufficiently. Does he have the necessary earning potential that you will require?
(Note: you could be working, too, of course.)
If your fiancé is only ever gonna have a CZ-level of income, are the other significant parts of your relationship able to fill the gaps that that low income might leave?
Not saying money is the one and only aspect of your future relationship. But realize that a significant number of marriages/relationships dissolve and they cite financial problems as a key contributor.