its fine... just propose without a ring and let her choose one that she likes that is diamond after...
you can got lab created diamonds which are just as strong as real diamonds and come at a far cheaper price, are environmentally friendly and has no blood or slavery involved... they are not in any way inferior and there's no reason to be ashamed of them or to avoid them... but be sure that is is truly a lab created or culture diamond and not cz being passed off as it...
you can also go the moissanite route which is what i did years ago and i'm still in love with my ring...
both lab created diamonds and moissanite are just as hard as diamond and both shall not get dirty or cloudy or change color like rubbish quality cz... they shall last forever like a "real" diamond... moissanite isn't trying to be diamond like lab created diamonds are btw, but they are clear like diamonds so i mention these two together...
however, if she doesn't want the traditional clear diamond appearance, there are plenty of gemstones she can choose from... ruby, sapphire, emerald, aquamarine, morganite, garnet, amethyst, topaz, tourmaline, peridot, citrine, tanzanite, tsavorite, spinel, golden beryl, heliodor, rose quartz, kunzite, poudretteite, scapolite, red beryl, rose de france, hessonite, apatite, ametrine, etc.........
some of these gemstones come in multiple colors... you should look with her to see what she prefers but do keep things like hardness in mind... also how she shall feel about the ring years from now... how it flatters her skintone... how it shall match or clash with every outfit, occassion, and mood which is why i personally believe lab created diamond or moissanite would be the proper option as they're the hardest choices, are universally flattering, and they suit every outfit and occasion and mood...
i also recommend platinum or palladium bands as they are superior quality to golds... avoid silver as it shall never hold up for life...