A video revealing Mega Sableye has been uploaded on the official Youtube channel. Sableye retains its Dark/Ghost typing and apparently uses the gem on its chest as a defensive barrier. That should mean that its defensive stats are increased once it Mega Evloves. You can watch the video below:
Sableye is a Dark- and Ghost-type Pokémon. Mega Sableye has concentrated all of the energy from Mega Evolution into the jewel which adorns its chest. Once Sableye has Mega-Evolved, this jewel will swell and become detached from its body. Mega Sableye uses the jewel as a powerful defensive tool in battle. From behind the shelter of the jewel, it can readily exploit the opponent’s weaknesses and take advantage of any openings. It is said that the jewel hardens into such strength that no attack can even make a scratch!