A picture would help.

Rambutan seeds are about the size and shape of an almond, and just as soft. Kind of a greyish beige color.

Lychee seeds are shiny dark brown and hard, kind of elongated or egg-shaped, but with a flat whitish area on one end, where the fruit attaches to it.

Sadly, I did not take a picture before it became swollen, and now it is unrecognizable. The seed coat (I believe) is crumbling/peeling away, and the seed is quite lumpy and fat now... but it was round like a nickel, but slightly flattened - like if somebody took a shiny gray 1/2 - 1" ball and flattened it a little. Thanks for the nice descriptions of rambutan and lychee seeds - I think I can rule those out now.

Thanks!

I have no clue what that would be

I have found it!!! :) It's a nickerbean, which I must have picked up in the Virgin Islands. Depending on which trip that was, it's got to be at least 5 years old now... but maybe it will germinate.

If anyone's interested, here are some links with photos of the seeds & plants:

http://www.seabean.com/guide/Caesalpinia_bonduc/

http://www.botgard.ucla.edu/html/botanytextbooks/generalbotany/plantarmature/a0603tx.html

Thanks for all your help!