It first appeared in chapter 34, when Ryoma acquired the new blood slime. (Yapping ahead)
Whilst trying to feed the slimes with the leftovers from the butcher, he found out that one of the slimes evolved into an Iron Slime, and from there, he experimented more, eventually leading him to inject magic onto slimes and finding out which ones had an affinity for it. In the end, he found out that 3 slimes had an affinity for magic, and more specifically, 2 of them liked earth magic, and the 3rd one liked dark magic. They make yet another appearance in chapter 36, and again in the recent chapter with the Charcoal Slime.
Thanks, I love when he gets a new slime. I'm gonna go reread those chapters :]
Maybe it's because I haven't read this in a while but the earth slime seems new to me. Can anyone tell me what chapter he got it?