Clay
Mentioned in Troll Bridge as Mica's fat merchant brother-in-law, this troll lives in Bitter Pike, and convinced his brother Chert to replant Cutshade Forest so that they could re-sell it. It is insinuated that Mica gives Cohen the Barbarian his address at the end of the story; I'm sure we can all guess why.
On Roundworld, clay is a naturally occurring material, composed primarily of fine-grained minerals, so this does qualify as a "rock" name.
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