Ice Ic (pronounced "ice one c" or "ice icy") is a metastablecubiccrystalline variant of ice. H. König was the first to identify and deduce the structure of ice Ic. The oxygen atoms in ice Ic are arranged in a diamond structure and is extremely similar to ice Ih having nearly identical densities and the same lattice constant along the hexagonal puckered-planes. It forms at temperatures between 130 and 220 K (−140 and −50 °C) upon cooling, and can exist up to 240 K upon warming, when it transforms into ice Ih. Apart from forming from supercooled water, ice Ic has also been reported to form from amorphous ice as well as from the high pressure ices II, III and V. It may occasionally be present in the upper atmosphere and is believed to be responsible for the observation of Scheiner's halo, a rare ring that occurs near 28 degrees from the Sun or the Moon.
Ordinary water ice is known as ice Ih, (in the Bridgman nomenclature). Different types of ice, from ice II to ice XVI, have been created in the laboratory at different temperatures and pressures.