Zircon is a silicate of zirconium corresponding to the formula ZrSiO_{4}, but uranium and the rare earths are generally present in small quantities. The aurora-red variety is known as hyacinth or jacinth, and the term jargoon is applied to the other transparent varieties, and especially to the yellow stones. The most attractive colours shown by zircon are leaf-green, golden-yellow, and deep red. Other common colours are brown, greenish, and sky-blue. Colourless stones are not found in nature, but result from the application of heat to the yellow and brown stones. The name of the species is ancient, and comes from the Arabic _zarqūn_, vermilion, or the Persian _zarqūn_, gold-coloured. From the same source in all probability is derived the word jargoon through the French _jargon_ and the Italian _giacone_. Hyacinth (cf. p. 211) is transliterated from the Greek ὑάκινθος, itself adapted from an old Indian word; it is in no way connected with the flower of the same name. The last word has seen some changes of meaning. In Pliny’s time yellow zircons were indiscriminately classified with other yellow stones as chrysolite. His hyacinth was used for the sapphire of the present day, but was subsequently applied to any transparent corundum. Upon the introduction of the terms, sapphire and ruby, for the blue and the red corundum hyacinth became restricted to the other varieties, of which the yellow was the commonest. In the darkness of the Middle Ages it was loosely employed for all yellow stones emanating from India, and was finally, with increasing discernment in the characters of gem-stones, assigned to the yellow zircon, since it was the commonest yellow stone from India. [Illustration: FIG. 78.—Zircon Crystal.]