Regarding using SAND:
You need to be careful when using sand to make sure that it does not have any salt residue in it. I don't know how you do this, perhaps you have to wash the sand first. But, I found out the hard way when I was making a slew of pincushions to sell at our guild's quilt show boutique. I must have sewn about 20 of them and filled them with the sand that you get from the hardware store. It was inexpensive, the sand weighed down the pin cushion and it would keep the pins sharp - Right? WRONG! Apparently there was salt in the sand and they eventually started pitting my pins and then they started to rust. I don't live in a humid area, so the only reason I could come up with was salty sand. Fortunately, I didn't sell very many, but the one's I gave away to friends I had to replace because their pins started to pit and rust, too. I used crushed walnuts as a stuffing this time.
My favorite pincushion is an oval one made out of sheep's wool.