I hear you on this, been fighting him all day and not one gem. I wish they would implement a trade system for these rare materials, at least a gem for gem trade. It's annoying when I fight Zinogre 2 times and get 3 Jaspers, but I fight Brachydios 20+ times and not one gem. --- http://s1.postimage.org/fm1o7jxqn/secret.jpg http://i.imgur.com/ezujF.jpg
Anyhow, if they follow the same rule as plates, its 3% to get one from rewards, 3% from carving tail, 2% to get one from breaking head (?), another 3% chance from capture rewards, and finally 1% from shiny drops (slime puddles sometimes leave them).
One of my gems did come from break rewards on a kill, so im assuming the head. Also i did get one from a slime pile. --- Apparently OP for his entire argument does in fact put his fingers in his ears and go "LALALALA" the entire thread. Fascinating.
Pretty much, only it's 2% on all the carves, and 2% in capture rewards. Not sure about kill rewards, guide doesn't list gem under any break parts though. --- http://s1.postimage.org/fm1o7jxqn/secret.jpg http://i.imgur.com/ezujF.jpg