When it comes to buying opals there are some inportant points to consider.
Each stone is different so you need to pick a stone that you feel suits you and your personality. Not just pick one that looks pretty or big.
It is very much a matter of taste and feel. Only you can pick an opal that suits you also.
You should also look at how you are going to use the stone. Is it going to be a fashion accessory, worn on special occassions only, or will it be for everyday use?
if you are going to have the stone set in jewellery such as a ring or pendant for example, the size and stuypoe of opal is very important. Check out the types of opals available and the colors. Stone can go from red through to white, black, orange and blue, the most common.
Check the brilliance. It should be excellent and show off the colors within the stone.
Every stone should carry a certificate of authenticity. This ensures you get an authentic stone and guarantees a resale value should you wish to sell it at any stage. Also a vital necessity for any insurance claims.
Check a stone that you have selected. Hold it up to the light and view it in natural light if you can. Inspect it for any cracks or signs of wear. The dealer from whom you buy it should guarantee that it is free from cracks and should replace it if a crack is later found to exist. He should also inform you if there are any cracks. A cracked opal has very little if any value.
lastly when buying opals off the internet one needs to select the dealer carefully and ensure they are willing to offer a guarantee that the stone being offered is exactly as displayed aa advertised and will take the stone back if found to be different.