OPAL

Opal is an hydrated Silica and is a gemological oddity.  While consisting of structured layers of spheroids of the mineral Crisobalite suspended in a silca gel it is generally considered an amorphose (non-crystalline) gem.
It has many varieties, but can be divided into three broad groups.
Precious Opal is characterised by a rainbow-like play of colour, which changes with the angle of observation, called Opalescence. Varities depend on the body colour and can be referred to as Black or White Opal.
Fire Opal is commonly red, yellow to orange in colour and does not usually exhibit a play of colour.
Common Opal is mostly opaque and includes varieties such as Agatised Opal, Hyalite, yellow Honey Opal and the dendritic Moss Opal.