Mine are up quite high. It was easy to see the green jewel from down below, and the other one appeared to be dark blue. So I climbed up to get a better look. And I discovered they do NOT have jewels. It is actually the color of the cape that I am seeing in the empty hole, and it looks just like a jewel is in there. Funny Windstone didn’t put in a jewel; it appears to be designed for one. So your Cat Wizard is fine.
I do wonder if a jewel was put in them at one time or another. I have a second Emerald Cat Wizard packed away just in case something happens to my first one, but I don’t want to dig him out to check.
As for putting in a jewel, changing a piece usually detracts from its resale value. In this case, who knows? People might assume it is supposed to be there if you still want to put one in.