Hydrous silica SiO₂.nH₂O associated with the chalcedonic (see chalcedony) varieties of silica. A layer of water molecules trapped near the mineral surface causes the iridescence (opalescence) which is a diagnostic property of opal; sp. gr. 1.99–2.25; hardness 5.5–6.5; amorphous; colourless, or milky-white to grey, red, brown, blue, green, to nearly black; resinous lustre; normally massive, but can be stalactitic, botryoidal, and also in veinlets, the various varieties depending on the amount of water contained in the mineral, which can vary from 6% to 10%; no cleavage; conchoidal fracture; normally deposited at low temperatures from silica-bearing waters, and occurs as fissure fillings in rocks of any kind, and especially near geysers and hot springs. The variety known as precious opal has a milky-white or sometimes black body colour which exhibits a brilliant play of colours, usually blues, reds, and yellows. The colours can often disappear with the loss of water when the mineral is exposed to air.