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.
Gronckle Iron is made from an unknown combination and amount of rocks and ore, which were melted inside of Meatlug's stomach and expelled as a reddish-orange lava. Once properly shaped and cooled by Gobber, the resulting metal takes on a silver sheen and is found to be lighter and stronger than pure iron. Though the exact recipe is unknown, a sword made from Gronckle Iron is strong enough to cut through an iron blade with surprising ease.
It is possible that Gronckle Iron is a high carbon steel without slag and detrimental impurities, since high quality steel is much shinier and stronger than crude iron. Due to the fact that Fishlegs has no idea what Meatlug ate to create the alloy, the limited number of weapons and tools that Gobber had forged are the only ones made from Gronckle Iron. This includes a number of swords, maces, axes, helmets, Hiccup's special shield and a pair of earrings for Meatlug (which she promptly ate...)