The 18 most probably stands for 18K. My guess is that for the stones to be out of what appears to be intact mountings, they would have to be stones that would possibly "disintegrate" or dry out over time, and fall out of the mountings. This would be probably opals. At this point they would be worthless if found.
The mounting is excellent though, and if mine, I would have some precious stones reset into it. Mounting is probably 1860's-1910. Someone earlier mentioned 1600-1700. Not this old, as stones were poorly faceted then and did not usually have an open back setting. This ring does, and the style woulf date it to 1800's, at the earliest.