Gronckle Iron

Gronckle Iron is a new metal alloy that was inadvertantly discovered by Fishlegs, Gobber and Meatlug.

Objects
Here are the objects that were forged out of the Iron:

1) Fishlegs' sword

2) Meatlug's earings

3) Hiccup's shield.

4) Several other objects belonging to multiple Berkians

Discovery
During episode 'The Iron Gronckle' Fishlegs and Meatlug were in a funk over how slow they flew 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 tickled 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.

Properties
Gronckle Iron is made from a combination of rocks and ore, which are melted inside of a Gronckle's stomach and expelled as a 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.

In Night of the Hunters Part II, 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. This differs to 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.