the answer to that is..... You're not! It's a test!
we women do this to you to see how well you know us. :oP
ok, what does she have just now?
Gold - silver?
white gold - yellow gold?
what stones does she have?
what colour clothing does she favour? - I wear lots of black so tend to wear white gold with diamonds or blue topaz (my birth stone)
Does she have an allergy to fashion jewellery? if so, don't buy anything less than gold, you'll be found out when the rash appears!
does she wear holiday jewellery (big clangy stuff) or delicate classic pieces?
hope this will get you started.