Zipporah took a sharp stone. Literally "a stone." Stone knives were commonly used in Egypt for making the incisions necessary when bodies were embalmed, and were regarded as purer than iron or bronze ones.
Joshua ordered the preparation of stone knives for the circumcision of those born in the wilderness (Joshua 5:2); and the Jews seem to have used stone for circumcision for many ages, though before the compilation of the Talmud they had changed their practice.
Cast it at his feet. Not, certainly, the child's feet, but her husband's, to whom at the same moment she addresses herself. A bloody husband. Literally, "a bridegroom of blood." The words are clearly a reproach; and the gist of the reproach seems to be that Moses was a husband who cost her dear, causing the blood of her sons to be shed in order to keep up a national usage which she regarded as barbarous.