Kamasutra 7.2.7

7.2.7

dārumayāni sāmyataś ceti vātsyāyanaḥ/

Or, he may make use of certain Apadravyas, or things which are put on or around the lingam to supplement its length or its thickness, so as to fit it to the yoni. In the opinion of Babhravya, these Apadravyas should be made of gold, silver, copper, iron, ivory, buffalo's horn, various kinds of wood, tin, or lead, and should be soft, cool, provocative of sexual vigor, and well fitted to serve the intended purpose. Vatsyayana, however, says that they may be made according to the natural liking of each individual.