The answers you've received are excellent. Sometimes it's also best just to use a common adjective for those reasons, so you aren't misinterpreted. She's cute / pretty / beautiful / gorgeous....

What context do you want to use them in? If it's in literature or poetry or a very poetic love letter you can make any of those sorts of romantic comparisons but it would be very weird to say something like that directly to or about someone, even if she was your girlfriend :)

Oh this is a difficult question. The reason we have generic terms of beauty is so people don’t feel awkward when they hear them. For example if I just met someone and said they were as pretty as a rose, they probably wouldn’t think I was being too intimate or intrusive and take it as a compliment. But if I say something really specific like, “your eyes are the bluest sapphires hanging from the stars on a moonless night” to someone I don’t know very well, it comes across extremely creepy. That’s why the generic ones are best and used all the time. If you know this person well though, I would recommend comparing their attributes to things you think are beautiful and maybe some things in your relationship that both of you like. This will make your compliments unique to you and that person and will be greatly appreciated.