Hey, I've got a question. I'm playing Emerald and I'm in a tower called Mirage Tower. Am I lucky to see it? Is there something at the top?

No, it is not incredibly rare, it just appears and disappears when you either return to the route it is on or leave the route. The top of the tower has the Claw Fossil and the Root Fossil (Anorith and Lileep, respectively), but you can only get one. D:

You can get the other one in the Desert Underpass after beating the Elite Four.

There are also Ditto and Loudred in the underpass. The underpass' location is at the fossil maniac's house, west of Fallarbor town.