I'm completely guessing here, maybe a tamarind seed that's been exposed to moisture/water for some time. Perhaps the restaurant used it for the salad dressing?

Whoops, the pics have not uploaded yet.

These are the seeds:

Here they are after soaking for about 12 hours or maybe more (the outer shells were nicked to speed up the process), the outer skin became soft and peeling (sorry, have no better picture to show it):

Does your nickel say 1963? If so, it's a keeper! (Before the composition of metal was changed.)

Was it a Greek salad with kalamata olives?

Unlike quarters and halves that had silver content through 1964, only "war" nickels from 1942-1945 contain silver.

The pic of seeds in your hand reminds me of tamarind.

Or did you have a house salad? Perhaps it flew on your plate while the chef was making someone else a Greek salad.Don't think tamarinds or dates would be good in a salad.

Tamarind can be used in sauces - including for salads and the seeds are not toxic so not critical to remove. Although they are hard like rocks. I like a Thai tamarind candy that leaves the seeds in. Look up "tamarind seeds" at Google and see what you think.

Fatamorgana, the seeds in my hand are tamarind seeds. The OP picture reminded me of it, so I posted it as comparison. I happened to have saved some seeds a while back.

Yes I've heard of tamarinds being used in salad dressing and also savory Indian and Mexican dishes.It could be tamarind,but OP E C hasn't chimed in to provide any more background info.I had tamarind candy that had Chile pepper in it that was a little strange at 1st taste,but became addictive.The size of pit next to nickel would fit with tamarind.

Hi again and thanks for all the feedback! I think tamarind could be a good guess. Are the seeds about the same size as cherry pits? I live in Southern California and though it wasn't a Mexican place, tamarind is used in a fair amount of dishes here. It's definitely not an olive pit or anything like that.