a generally vernacular building material

Dave - interesting word and etymology on tabby, but I've no more to contribute on that point, so, by your leave, I'll take the thread in a completely different direction.

Your use of "vernacular" here caught my eye, as I've always understood it to be a term applied to language, per the first three senses given by AHD:

1. Native to or commonly spoken by the members of a particular country or region. 2. Using the native language of a region, especially as distinct from the literary language: a vernacular poet. 3. Relating to or expressed in the native language or dialect.

However, it's clear that you were using it in a sense that makes great sense, but that was new to me:

4. Of or being an indigenous building style using local materials and traditional methods of construction and ornament, especially as distinguished from academic or historical architectural styles.

So that was interesting - but what I really found interesting was the etymology:

From Latin vernculus, native, from verna, native slave, perhaps of Etruscan origin.

Nice use of the Etruscan in your post, Dave.