Any plant that leaves a stain on your hands can be used as a wood stain. I would recommend you put your shells (or whatever, beets, blackberries, raspberries, onion skins, etc) in a saucepan, cover with water, and boil, then let steep. The more materials and the longer you let it steep, the more intense the color. It will not be light fast, and will fade over time, but not enough to worry about.
Strain through cheesecloth or muslin, and apply with a brush or cloth. Sand lightly and put on more than one coat, and then seal with oil or maybe a wax finish.
Calendargirl, you don't need vinegar for plant colors. For some applications, you do. I did a two-part stain for my last bow using black tea boiled in water, and cider vinegar I had soaked a ball of steel wool overnight in for the second part. I brushed on the tea water, let it sit for a few minutes, and then brushed on the steel wool / vinegar mixture. The wood began darkening immediately, and looked great, a rich brownish-gray.
Dane