Welcome to the forum Pinzi. Firstly, you are correct, this is what is known as a crystal opal, it won't necessarily have gone cloudy through storage if it's Australian opal. By getting it polished it will bring out the colour and make the stone look clearer. If you dip it in water, it will give you an idea of what it will look like polished. Your local lapidary club may be a good stop to see if anyone there works in opal. Secondly, as far as valuation goes, I would wait until it's polished as this, if it's done properly, will add value to it. Your photos don't show a lot of colour in it, it may just be the photos, but it it looks better in the flesh, it should be more valuable. Again, the lapidary club maybe able to help you with valuation as would a professional gemmologist, but you will have to pay for their services. It may turn out to be more valuable as a keepsake of the person who left it to you. Either way, you won't be able to retire on what it would bring! I hope this helps you, as you should understand, it's extremely hard to value something via a photo, particularly opal.
