A bookbinding tool. It looks like an rounded ivory tongue depressor with one pointed end. You use it to score paper for folding and for creasing folds that you have made, thereby saving your fingernail. SEF has pointed out that they are also good for folding business letters if you would like to avoid the impression that you have folded said letters with your foot.
It's one of those things that sounds useless until you use one. Available at finer art stores everywhere for around $5.