we struggled a bit when it was my mum and dads but we ended up giving them a night away at a hotel which I know is not really anything to do with 'pearl' you could give something like this: http://www.apuldramro...ARY+whitston-rose.htm or I am sure online you could find a nice photo frame or album with a pearl covering? Sorry nothing very original. My mum (and I) not very keen on pearl so did not make much effort to find something with it.
You could always buy something in Mother of Pearl. OR an invitation to a seafood restaurant as pearls come from oysters, if they like that kind of thing. (One New Year's Eve, we found a pearl in an oyster we opened!).
Or just 30 somethings of what they like. HTH
If you don't want to give them something made of or decorated with peals why don't you get them something beginning with each of the letters of pearl (and mark the gift tags accordingly) - might give you more scope to think of something original (unless you think this idea is too cheesy for words!).
Do they have a garden, how about a sun dial, bird table or bird bath or just vouchers so they can choose some plants, or a tree, we bought a tree when each of our grandchildren were born