Suggestion: Give gabbro men the ability to slowly "swim" through walls & floors of natural unmined gabbro, thereby allowing the potential for mineral-based creatures to literally walk right out of the walls into the heart of your fortress. This would:
a) make gabbro men a highly unique (and therefore interesting) enemy,
b) make fortress defense more dynamic, as simply walling in would no longer be such an easy & viable strategy, and
c) teach players what the phrase "living rock" actually means.

The idea that the gabbro men could be stopped (or at least discouraged) by smoothing--and especially engraving--the walls/floors makes sense, as this makes the walls less natural, and gabbro men can only travel through unaltered gabbro. But then again, I also like the idea of gabbro men stepping through an engraving, and having the engraving transfer to the gabbro man, so he's "wearing" it like a decoration. (This does not destroy the masterpiece artwork, so the engraver would only be mad about the loss if the ☼«gabbro man»☼ is then killed.) This could theoretically open the way to a peace treaty between the dwarves & gabbro men, in which the dwarves allow the gabbro men to freely visit and get decorated.

Other types of mineral men could be added, too, one for each major layer stone, with each type able to traverse its own native mineral.