What would be an appropriate 50th wedding anniversary gift; they are my aunt and uncle not parents

Gold is traditional and contemporary for 50 years of marriage. It doesn't have to be real gold, but anything gold coloured would do nicely. How about a white and gold album with some pictures you have assembled in it.

Tick

Depending on your budget there are a lot of options - For people who are well settled into their lives, I like giving some sort of experience as a gift like theatre tickets or a gift certificate to their favorite restaurant (or one they've never tried). Perhaps find out where they went on their honeymoon and maybe along with some other family members, you can all chip in to send them back there for a couple of nights (or even one night if it's close). Simpler options might be flowers, or depending on where they live, perhaps something they can plant in their yard like a rose bush.

I was recently at a 50th anniversary party and it was planned in advance that people would replace things they would have gotten as shower gifts when they married. It was a lot of fun - the couple got new towels and kitchen utensils and so on - but to pull this off, it helps to have someone who knows what they need (like one of their children).

Nothing says "Golden Anniversary" like gold. If all you can afford is a card, sign it in gold leaf. You can get gold leaf writing kits at most craft stores fairly inexspensively.