Yes, I've had this happen before, and yes it is a bug.
You get the "Garnet" dialogue string every third time you talk to any "non-special" (e.g. named) townsperson. It works for any good cleric, inside the tavern or out.
The supply appears to be infinite. If you find three people together, you can go around in a circle, and have the same individual give you scores of garnets. I tried it once, and stopped at a hundred.
