with regards to ruby, diamond, and other precious gem, mining the part I find enjoyable is most of these places are off the beaten path and a day spent in the country playing in the mud can be rewarding for lack of distractions of everyday life
As far as a cave that you can simply take an elevator down to and walk through on paved sidewalks, it is one of the best in the world. (There are also other portions for the more adventurous. Some that are completely underwater and require scuba gear.)
I think there are even places you can buy a bag of dirt to be shipped to you.
Another interesting thing you can do is buy unsorted/unwashed roman coins from europe. There are techniques to soak and reveal the face of the coins. Where else can you uncover 2000 year old human artifacts?