Just as an fyi:
My family enjoys doing this, and we usually go to the Sheffield mine. Last summer I found a 30 carat sapphire, and took it to have it made into a pendant for the wife. The jeweler told me that it is extremely unusual (like maybe 10%-20% chance) to find a stone that has any value, or that can be cut. The reason is that at some point in the area's history, there was a volcanic event that formed the gems. There was then a subsequent volcanic event. This allegedly damaged the existing stones, so that, though they look good when you dig them, when a jeweler tries to cut them, they will be shot through with tiny holes like a sponge, and will often shatter during the cutting process.
Fortunately, my stone was one of the rare ones, and the wife now has a necklace worth about $600-$700.
Don't know about the veracity of what he told me, but I thought I'd share it.