Your gem would be chalcedony (kal-sed-oh-nee).
It would be a little chinstrap/goatee type of thing and your weapon would be a giant war hammer (size can vary but max length would be just longer than your body). Special gem power is uhmmm, I don’t know. The image above makes me think of clouds so maybe you can turn your body into a gaseous form. If I think of something better I’ll make an addendum! 

Oh dang I love this! If it was a chinstrap thing I would wear a helmet like 24/7 to fit with the style and I would just slam my jaw against shit lololol! 

Also hell yea to turning into a gaseous form, IMA KNOCK YOU OUT