First of all, congratulations!!! In another lifetime, I worked in upscale jewelry retail - so I do know my diamonds.
Do a Google primer of diamonds and learn all about cut, clarity, colour (carat, is IMO, the most subjective of the four “C’s”). But…don’t limit yourself to diamonds. Some women might like a coloured stone, i.e. sapphire, emerald or ruby. Buy what you think she’ll like (if you can get a sense of her, maybe find out her preferences. But, insh’Allah, she’ll love it, because it was a gift from you.