Volcanic tuff was a key material in the Roman invention of concrete, something that modern concrete, or "Portland cement", lacked. With tuff, not only would Roman concrete be waterproof, it would also harden as it would get older.
For the concerns of both geologists and construction designers, how common is volcanic tuff?
As an example for what I'm looking for, this map shows grey veins of coal. Is there a similar map showing global distribution of volcanic tuff deposits?