It has the appearance of Chalcedony but also has the appearance of some Opal I have seen! No way to determine a value without knowing for sure what it is. If you don't have any type of Rock/Gem Shops in your area, I would suggest taking to a Geology Department of a nearby University and they should be able to tell you what it is. It might help if you could take some more pics showing all sides of the rock/stone and post them here as they may be helpful in determining what it actually is.
Frank