the difference between a "crowd" and a "mob" is one push
Reading the rules, I found the description of Silver to be a Resource but not a "building Resource". The Stonemason says that a player pays 1 taller for a "building Resource" from a worker. Does that mean he cannot pay for Silver?
My reasoning for No is: every other Resource can be applied directly toward erecting a building, but Silver needs to be processed first in the Forge. So the Stonemason is not really buying a specific Resource.
We play that yes, the silver can be taken if it's on the worker card.
Page 4 under "Equipping worker cards" says in part "When a player has played a worker, he takes the 3 resource pieces depicted on the card from the general stock (carts and riders) and places them on the cards. No exception is mentioned for the silver bars, in fact they are implicitly included by the word "rider", which is where the silver bars are taken from.
Stonemason says in part, "With the stonemason, a player can buy one piece of resource from the card of any worker who plays a worker in that round." Again, no exception is mentioned for the silver bar.
I think if there was a special rule for the acquisition of the silver bars, it would have been stated.
Talking Aviator says: “It is the time you have wasted for your rose that makes your rose so important.”
I agree that the Stonemason can buy a Silver bar from a worker's card, but only from the Worker card that is explicitly stocked with a Silver bar (and 2 wood pieces). The Stonemason buys the resources that are laid out on the card, not resources from the player's stock.
When the Worker plays the card that has the 2 "wildcards" (1 stone and 2 "wilds"), the Worker is restricted to stocking the wilds with sand, clay or wood - the "Building Resources" as Osiris mentioned in the original question. The Worker controls which Building Resources are available in this situation...