Delhi and Jaipur are known for meenakari, the jewel coloured enamel work on gold, silver and since the 1980s on copper. The art involves the fusion of coloured minerals, such as cobalt oxide for blue and copper oxide for green, on the surface of the metal giving the effect of precious stone inlay work. The particular mode employed is known as champleve where the metal is engraved or chased in such a way as to provide depressions within which the colours can be embedded. The colours are applied in order of their hardness, those requiring more heat first and, those less, later.