Diamonds you goose.
Oh, OK, if you're going to be tight fisted about it then
Diamond cut crystal - maybe a decanter, or set of wine glasses.
A 'diamond' themed gift hamper - boxes and packages shaped in the shape of a diamond.
Diamond patterned place mats or vase or whatever (sorry - gets a bit vague there 'cos I don't know their 'style'.
My grandparents were married 75 years. What ever you do, make sure what you give is what you can cope with when they decide to give back all their 'stuff'. My 95 year old grandmother has been doing this for the last 5 or 6 years and people are starting to regret buying her and her husband some of the stuff they thought was a good idea at the time.
