Ice is a solid phase, usually crystalline, of a non-metallic substance that is liquid or gas at room temperature, such as ammonia ice or methane ice. However, the word 'ice' normally means water ice, technically restricted to one of the 15 known crystalline phases of water. In non-scientific contexts, it usually describes ice Ih, which is known to be the most abundant of these phases. It can appear transparent or an opaque bluish-white color, depending on the presence of impurities such as air.