Just an out-of-topic question about jewels, gems & their value in gold.

I know this doesn't have much of anything to do with Spelljammer, but
anyone's help would be appreciated.
I'm trying to figure out if there is some sort of chart, or table or chapter
in some AD&D/D&D/d20 book that mentions how much gold jewels & gems are
worth. Namely because some PC's I know found a watermelon sized-jewel & want
to know how much they can get for it. If it was normal gems I guess they
could carry it around, but this thing is pretty huge.

Any help is greatly appreciated.

-Chris