Well if she likes silver and not diamonds, it all depends on your budget. Bear in mind an engagement ring has to be long wearing, so silver is not a good idea. It is too soft, while gold is cool, but it the colour isn't quite the same and it will need re plating with rhodium every so often. So unfortunately that leaves platinum, the strongest, shinyest bestest and priceyest of all metal. There is also palladium, for wedding rings, it's almost the same, but not as rare, so it is cheaper. However there are not many palladium rings with stones in yet, as it is as hard to work with as platinum.
Stones wise, go for one of the big ones, emerald sapphire, ruby maybe, or tanzanite, which is easily one of the pretties stones around.
Next be aware of settings, if you go for gold, make sure it isn't tension set, or the stone will eventually fall out. Claws are best, make sure they are not to pronounced or she'll catch them on things.
That's about it for the ring, but it's important. (My mother is a jeweller so I know plenty about the shiny things.

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