and gold are most obviously yellow. Phosphorus, arsenic and antimony have allotropes which are yellow or whitish-yellow; fluorine and chlorine are pale yellowish gases. Many crystalline chemical compounds, such as 2,4-Dinitrophenol, are yellowish in color. Pigments Yellow ochre (also known as Mars yellow, Pigment yellow 42, 43), hydrated ferric oxide (), is a naturally occurring pigment found in clays in many parts of the world. It is non-toxic and has been used in painting since prehistoric times. Indian yellow is a transparent, fluorescent pigment used in oil paintings and watercolors. Originally magnesium euxanthate, it was claimed to have been produced from the urine of Indian cows fed only on mango leaves. It has now been replaced by synthetic Indian yellow hue. Naples Yellow (lead antimonate yellow) is one of the oldest synthetic pigments, derived from the mineral bindheimite and used extensively up to the 20th century. It is toxic and nowadays is replaced in paint by a mixture of modern pigments. Cadmium Yellow (cadmium sulfide, CdS) has been used in artists' paints since the mid-19th century. Because of its toxicity, it may nowadays be replaced by azo pigments. Chrome yellow (lead chromate, ), derived from the mineral crocoite, was used by artists in the earlier part of the 19th century, but has been largely replaced by other yellow pigments because of the toxicity of lead. Zinc yellow or zinc chromate is a synthetic pigment made in the 19th century, and used by the painter Georges Seurat in his pointillist paintings. He did not know that it was highly unstable, and would quickly turn brown. Titanium yellow (nickel antimony titanium yellow rutile, ) is created by adding small amounts of the oxides of nickel and antimony to titanium dioxide and heating. It is used to produce yellow paints with good white coverage and has the LBNL paint code "Y10". Gamboge is an orange-brown resin, derived from trees of the genus Garcinia, which becomes yellow when powdered. It was used as a watercolor pigment in the far east from the 8th century – the name "gamboge" is derived from "Cambodia" – and has been used in Europe since the 17th century. Orpiment, also called King's Yellow or Chinese Yellow is arsenic trisulfide () and was used as a paint pigment until the 19th century when, because of its high toxicity and reaction with lead-based pigments, it was generally replaced by Cadmium Yellow.