Hi, that item is obtainable by using an obsidian Knife in this:
You can get a raw watermelon tourmaline from Drumeone of The Order of The Lion quest bosses. And the four minibosses from The Grimvale quest- An ancient Feud:
Srezz Yellow Eyes
Katex Blood Tongue
Utua Stone Sting
Yirkas Blue Scales
I don't know the chances, a friend told me its 1/100 you get the gem, I don't know if he was just guessing but I do know you can also get another variation of it (under the Raw one):
EDIT: I didn't know either so I asked a friend who got some of the gems and he explained me, I just used mine (Which I got from Drume) and this was the result :(