Ultramarine is used to describe things that are very bright blue in colour. an ultramarine sky. a very bright blue colour (ultramarinus ; because the substance it was made from was originally brought from Asia)
A blue pigment formerly obtained by powdering lapis lazuli, but now produced in large quantities by fusing together silica, alumina, soda, and sulphur, thus forming a glass, colored blue by the sodium polysulphides made in the fusion