I would go back asap.
Work that area again...and again and when you worked it over, work it again after a rain event, work slow. Betcha there's more out there...
Tip - you mentioned black sand? that's a good thing. Take a flat shovel, dig down as straight as you can, I'm guessing you will find another black layer under this black layer then beach sand, the OREO affect...you need to dig down until you see that black layer with virgin beach sand underneath it, there may be a few layers of black sand...that last black layer should be where more will be found. It may be several feet down or just a couple...but should produce.
What what the area it came from? coastal? park? field?
As to storing it, hmmm to clean it or not to clean...that is indeed the question.
For now just mix baking soda into a small glass of water...soak cob in the soda bath...just let it sit...don't brush it or rub something on it...just let it soak for an hour, then remove, rinse with fresh water-distilled water...several times. Then dry it with a blow dryer (set low low low) don't heat it, don't use any cloth etc...then store it dry, away from humidity !!! moisture...
My dad found a 8 reales similar to the way your cobs surface appears...we debated cleaning it...this is what yours may look even after a proper cleaning, some are just best left alone.
http://forum.treasurenet.com/index.php/topic,185219.0/topicseen.html
Enjoy it...
Trez