Here are the objects that were forged out of the iron that we see in the franchise:
- Fishlegs' Gronckle Iron Sword (forged in "The Iron Gronckle")
- Meatlug's earrings
- Hiccup's Shield
- Several other objects belonging to multiple Hooligans.
- Toothless' armor (forged in "Snow Way Out")
- Hookfang's armor (forged in "Edge of Disaster, Part 1")
- Hiccup's Diving Bell (reinforced)
- Hiccup's Diving Helmet (reinforced)
- Macey (as of "Twintuition")
- Viggo's Fire Sword
- Fishlegs' helmet
Fishlegs and Meatlug were upset over being slow as they fly and questioned their overall usefulness to the other Dragon Riders. Both went on an eating binge with Fishlegs eating handfuls of berries, while Meatlug ate an uncounted number of different rocks. Later on, they went to Gobber's forge in order to have him cure Meatlug of her stomach pains. When he tickled her on her underbelly, she belched out a huge amount of lava that was differently colored from her usual type.
When Gobber grew impatient over waiting for the lava to cool, he decided to forge a sword out of some of it, and discovered that the new metal was both lighter and far stronger than the best iron weapons and tools on Berk. This news quickly spread among the villagers with everyone wanting swords, maces, axes and other implements made from this new metal, which was soon dubbed as Gronckle Iron, as shown in "The Iron Gronckle".
The Dragon Hunters also discovered this metal and forced their captured Gronckles to make the iron, as revealed in "Night of the Hunters, Parts One and Two". It was at this time that Fishlegs discovered the formula to making Gronckle Iron.
The Dragon Riders learnt that pouring molten Gronckle Iron into the craters of the Edge volcano helped to stabilize it in "Living on the Edge". Hiccup and Fishlegs then came up with a concoction even stronger by mixing it with Death Song amber.
|“||Three parts limestone, two parts sandstone, one part iron ore, one giant spoon of hardened Gronckle lava, yeah! [src]||”|
|— Fishlegs exclaiming about the formula|
Gronckle Iron is made from a combination of rocks and ore, which are melted inside of a Gronckle or Thunderbottom's stomach and expelled as reddish-orange lava. Once properly shaped and cooled, the resulting metal takes on a silver sheen and is found to be lighter and stronger than forged iron. A sword made from Gronckle Iron is strong enough to cut through an iron blade with surprising ease.
The ingredients for Gronckle Iron were revealed to be three parts limestone, two parts sandstone, one part iron ore, and one giant spoon of hardened Gronckle lava in "Night of the Hunters, Part 2". This differs from the ingredients used in School of Dragons: sandstone, iron, and a mysterious black rock (called "spooky-nighttime-with-no-stars-or-moon-stone" by Snotlout).
It is possible that Gronckle Iron is purified iron (without the slag and impurities), with some carbon and nickel mixed into it, thereby creating a form of steel. This would make the alloy both lighter and stronger than crude iron and give it the metallic sheen.
Gronckle Iron has been proven to be extremely tough, as it has withstood many things. For example, it has been able to endure a full powered plasma blast, as Hiccup instructed Toothless to give it all he's got, as well as a close-range spine shot from a Deadly Nadder in "Bad Moon Rising". It also resisted an Armorwing's fire for several seconds as well as a Thunderdrum's sonic blast.
When bonded with Death Song amber correctly, the Gronckle Iron not only withstands intense heat but also intense pressure. If incorrectly bonded, the Gronckle Iron becomes incredibly unstable and explodes.
|“||He is, however, more than happy to help produce the Gronckle Iron needed to reinforce the structures, in turn making Berk stronger than it was before.||”|
|— Dragons: Titan Uprising|
As stated in the description of the Thunderbottom Berubble, Gronckle Iron can be used to reinforce structures, making them far stronger than they were before its addition. It is implied that this reinforcement enables the structures to withstand heavy impacts, particularly the impact of a Thunderbottom's tail.