Background Colour
It is important to know the type of opal. Solid precious opal is more valuable than doublet or triplet opals. Black opal is more valuable than boulder opal, which in turn tends to be more valuable than light opal. The darker the body colour, the more valuable the gem.

Dominant Fire Colour
The clarity of the colour is critical when assessing the value of opal. Red fire is the most rare colour, followed by green/orange, green/blue and blue. Therefore red fire opal is generally more valuable than a predominantly green opal, which in turn is more valuable than a stone showing only blue colour.    
However, brilliance and clarity of an open proportioned pattern are the main decision makers - a brilliant blue/green can cost more than a dull red; bright twinkling stars of a 'pinfire' pattern can cost more than a cloudy open pattern of similar colouration; or a brilliant, lustrous light opal can cost more than a lacklustre black opal.

Colour Pattern
Almost all opal displays some form of pattern. You will never find two opals that are the same. In particular, many of the top quality pieces of black opal from Lightning Ridge show their strength through a distinct recognisable pattern play of strong colours. Some of the various patterns are listed below.
The value of an opal is depends on many factors. Body tone, brilliance, pattern, colour bar thickness, faults, and the play of colour, play important roles in determining the value. Opal is generally examined and given a price 'per carat'. Other important factors include the quality of the cut & polish, and the size of the stone.

Body Tone
Body tone is one of the most important factors in the classification and valuation of opals. Body tone refers to the background or the 'underlying colour' of the opal, which ranges from black through dark to light. Generally opals with a black or dark body tone are more valuable than those with a white, light, or crystal body tone.  Black opal from Lightning Ridge is the most prized opal and may realise prices over $10,000 a carat.
The term crystal opal refers to the 'diaphaneity' (transparency) of an opal, not its crystal structure, and is defined as any type of opal which is translucent to transparent. (See image, below) Some crystal opal displays colour so intense, so dark, that the opal is referred to as 'black crystal opal.'

Brilliance
Brilliance refers to the brightness and clarity of the colours displayed by opal, when the stone is viewed face-up. This ranges from brilliant , bright , to subdued or dull.
The Play of Colour
The phenomenon known as the play-of-colour is the brilliant range of the full spectrum of colours caused by the diffraction of white light by the internal structure of orderly arrayed spheres of silica. Red (fire) opal is generally more valuable than a mainly green opal which, in turn, is more valuable than a stone showing only blue colour. Nature does not produce a red colour as often as it does a blue or green. Red colouring is caused by larger microscopic silica spheres, whereas blue is caused by the more common small spheres.

Faults
Faults which can detract from the value of a finished opal are many and varied. A crack in the face can render almost worthless an opal that otherwise might have been worth a considerable amount per cart. Crazing , i.e. many small cracks in the opal's face will relegate the stone to worthless. Sand and various other minerals can be found as inclusions in and/or under the colour bar, and in the potch of opals. Other faults include potch lines, webbing, (grey lines) and windows . The consistency of colours and pattern when viewed from different directions also has an influence - when a stone "won't face", the colour only shows through on certain angles and otherwise has little colour.

Conclusion
All the above factors are taken into account when valuing opal, however there is no substitute for experience. Truth be known, there is no true or set method for valuing opals, as each opal is extremely unique in terms of pattern, brightness, and colouring (unlike diamonds, which can be more accurately valued according to a set chart of colours, clarity, faults, etc.) Always ask for a certificate of valuation / authenticity, and get a second opinion from an experienced valuer if you are concerned about the value of a stone.