Is the secret to smithing Adamantine known only by the Technic League?

Question

Lately, I've been playing a campaign located in Belkzen territory, about a new ship falling down on Golarion and bringing a new heap of adamantine with it, a la Numeria/Starfall.

Skipping the potential cataclysmic aftermath of a mountain sized ship crashing at terminal velocity on Golarion (again), we're thrown in an rush to the adamantine, as there is plenty of gold to be made and scattered tribes of surviving orcs to be slain.

But recently, my group and I have been nagged by our DM about the slightest things in the campaign (mostly payback for the group not getting along OOC - another story). Last argument is: "your Expert smithing NPC does not know how to work adamantine , as it is only present in Numeria and the secret has not got out of the Technic League".

Now, I get around with the lore, so I would say it's wrong, as far as I know. A few counter-examples are:

1. The Adamantine Golem, from Bestiary 2, which is so expensive in adamantine that one needs to travel to the Earth Plan to gather enough to build the construct.
2. Adamantine weapons and armor in general. I can't remember the source, but there is a merchant in Last Wall (GM favourite nation ever) specialized in adamantine weapons. (Even give discounts to characters already using adamantine gear).
3. The novels City of the Fallen Skies & Reign of the Stars by Tim Pratt, explaining quite a bit about skymetal and adamantine .

Now, those are at most circumstantial evidences, since none exactly quote "Adamantine is used and worked all around in Golarion" and my GM can be quite stubborn.

So I'm looking for any additionnal sources pointing at the fact that Adamantine can be smithed by any quite skilled smith, coming from the Technic League or not.

Answer 1

Your GM is thinking about Glaucite

Glaucite is the material found when Divinity (the spaceship) crashed into Golarion, among other skymetals, and according to the information published under the Iron Gods Adventure Path, its forging technique is exclusive to members of the Technic League.

There is some info about it on Pathfinder wiki, under Adamantine:

Glaucite is a dark and dull grey alloy of iron and adamantine and is pretty much what is meant when 'Numerian steel' is discussed rather than pure adamantine. It is used in the creation of spaceships, hence its relative abundance in Numeria where the spaceship Divinity crashed during the Rain of Stars. Abstracting the pure adamantine from glaucite is prohibitively expensive, so rarely performed. Glaucite is equivalent to steel for weapon and armour manufacture in its properties, but tremendously more difficult to fashion—three times more expensive to do so—and it weighs 50% more than steel: it is, therefore, rarely used for armour or weapons except for affectation. In addition to spaceships, glaucite is the material of choice for crafting robot frames.

To answer the title question: No, it is not. From the same wiki page, we know that:

In the ancient Thassilonian Empire, adamantine was associated with transmutation magic, generosity, and greed, most likely because it was the most sought-after and valuable of the skymetals.

So, not only Adamantine has been around for centuries, abeit rare, the knowledge about smithing it is nothing new in Golarion.