שֵׁן shen "tooth, ivory". Hebrew does not use different words to distinguish "dead elephants' teeth used as decoration" from other live teeth, so at Amos 4:6 human teeth are being referred to, while at Amos 3:15; and 6:4 ivory used as decoration is intended. (Presumably though "couches" for human use, made of ivory might be possible, houses made of ivory were not! Hence I understand both "couches of ivory" and "houses of ivory" to mean "decorated with ivory".)