In certain conditions this structure is able to refract white light into the various spectral colours. Yes there are three main groups of opal - White , Black, and Boulder. These three groups also have a crystal translucent form. Generally light does not pass through the three main groups. The Black and Boulder opals are generally the darker coloured ones.
White opal is the traditionally percieved image, oval cut with a cabochon.
The opal types occur due to the type of topography in which the silica sedimented in. The opal's ability to refract light means that it can occur in almost any colour. Hence the primary benchmark for value is colour. The dark red coloured opals are the most valuable. The way the colour comes from the opal stone in the form of patterns also influences value.