During the 19th century it was discovered that by placing metal compounds into a flame each metal produced its own characteristic colour.
Passing the light from the flame through a series of prisms (called a spectroscope) would cause the light to split into individual wavelengths producing a pattern of coloured lines or ‘spectra’. These could be used to identify the metal.
The Streak Test
The colour of a mineral is not always the same as the colour of the mineral when it has been powdered, especially in the case of metals. An easy way to look at powdered minerals is to draw them across unglazed porcelain. The mark left is called the ‘streak’ and its colour may be used to help identify minerals. For example, Cassiterite looks black, but it’s streak is white.
Today, sophisticated analysis is used to identify metals.