Here is the rules quote for reference: 

The Blacksmith has obtained a legendary knowledge of their craft. They are the foremost specialist their field and are considered an absolute authority in all matters of Blacksmithing. A Legendary Blacksmith can create items of Legendary forging, giving the item properties of an almost magical nature. Legendary weapons and armour are immune to effects which will permanently destroy them. The creation of legendary armour and weapons will cost the smith 1000% of the original Raw Material value. Please note that Legendary Shields are exempt to the above cost. All Legendary shields cost 200 RM to produce.

Legendary crafted armour possesses superior strength and protection. Each piece of Legendary armour will also provide the wearer with an additional Armour Point for that location. Finally, Legendary crafted weapons and armour are the base material for any Blueprints a Blacksmith may create. Legendary items, once crafted, are so durable and resilient that they can resist the effects of ritual magic. Even the most powerful enchantments will quickly fade if cast on a Legendary item. An enchantment may last a few days or may only last 10 minutes. The duration is random and chosen by a Ritual Marshal. Legendary weapons and shields do not expire.

Page 80.

My question refers to the above stating that both legendary armour and weapons become immune to effects that would destroy them, however shields seem purposefully omitted. Thus, with the different shield types having corresponding thresholds ( 10 for wood, 20 for Iron-Bound, 30 for Metal. Page 89 ), would a Legendary shield break upon being struck by a single instance of physical damage that exceeds its threshold or is it considered to be a weapon, and therefore immune to that effect.