PYROPE (pronounced pirop)  , a deep red variety of garnet, named from the Gr . 7rvpw7r6r (fiery) in allusion to its colour . It is used, like almandine (q.v.), as a gem-stone, but may be distinguished by the absence of any tinge of violet in its colour and by its lower specific gravity (3.7 or 3.8, while that of almandine is 4.1 to 4.3) . The typical colour of pyrope is blood-red, though sometimes a trace of orange gives rise to a hyacinthine hue: occasionally the mineral becomes nearly black, as seen in the pyrope of Arendal in Norway . Crystals are rare, but cubic forms have been observed . Pyrope may be regarded as a magnesium-aluminium garnet (see GARNET), but it usually contains more or less calcium, iron, manganese and chromium; and the rich colour of the mineral seems due to the presence of some of the last three metals, though their exact condition in the mineral has not been determined . Pyrope generally occurs in grains embedded in peridotites (olivine rocks) or in serpentine resulting from their alteration, or it is foundthe earliest known is that of Homberg, prepared by heating a mixture of alum and finely divided carbon to redness in a closed tube . On opening the tube and emptying out the black residue (consisting of potassium sulphide, aluminium sulphate and carbon) it promptly catches fire . Many readily oxidizable substances, especially when very finely divided, have the same property . Metallic iron and cobalt, when prepared under certain conditions, are pyrophoric, as is also ferrous oxide . Spontaneously inflammable liquids are also known, e.g. certain alkyl metallic compounds, phosphorus dihydride, &c .