My grandmother owned a pair made of ebony, that were shaped much like scallop shells. I was surprised to find the etymology is from "chestnut" which comes from genus "castanea". I could never imagine my definitely proper grandmother dancing seductively.

castanets .pl.
5Fr castagnette (sing.) < Sp castaOeta, dim. of castaOa < L castanea, CHESTNUT: so named from the shape6 a pair of small, hollowed pieces of hard wood, ivory, etc., held in the hand by a connecting cord and clicked together with the fingers to beat time to music, esp. in Spanish dances