Mithril Insignia

The Mithril Insignia is given to prospective Armorsmiths as a symbol of their commitment to the Mithril Order.

Source
The Mithril Insignia is given to Blacksmiths as they start their quest to become a Master Armorsmith.
 * Alliance Blacksmiths get the Insignia from Hank the Hammer when they start the quest.
 * Horde Blacksmiths get the Insignia from Ox when they start the quest.

In either case, completing the quest demands that you surrender the trinket.