Opals are beautiful to look at and are considered by many to be precious. If you have ever seen one, you know that they aren't stones of a single colour, but tend to give off the reflection of many colours when the light hits them just right. This is caused by a consistent pattern of spheres within the stone that fragment the light and reflect it kind of like a rainbow. There are three grades of opals---common, noble and precious---and it is fairly easy to figure out how to tell them apart.
- Skill level:
- Moderately Easy
Hold the opal up to the light and look for colour. Valuable opals will have a reflection of one or more colours in them. If you do not see any colour or need a really bright light to see a small amount of colour, then it is called a common, or commercial, opal---the lowest grade. It is not very valuable. As far as colour overall, red is the colour wanted most, which makes it valuable even though it isn't as rare. Gold and orange are also valuable due to their rarity. Blues and greens are more common.
Look for fragments of other stones in the opal or some sort of backing or covering on the back of an opal while at the jewellery store. This signifies that the stone is a noble opal, or gem opal, and you should see colour in the stone in shade or light. In fact, its colour is better in shade. It is the second grade of opal and does have some value to it, but be careful that the jewellery store isn't passing it off as a top grade.
Look for colour and no backing when purchasing an opal. If you find this, you have what is considered the top grade of opal. It is called a precious opal or top gem. You will find no trace elements of any other mineral in these stones. They are solid all the way through and will have bright colours that do not go away when removed from light. In some cases, the colours will change in different lights.
Look at the colour of the stone itself. This doesn't change the grade of the stone, since that is based completely on the things above, but the stone's colour does change the value. Black is best and white is lowest with crystal and grey in the middle. However, a white opal can be more valuable than a black opal if the white opal has multiple bright colours and the black only has one.
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