Second episode of Defenders of Berk
While out chasing Outcast Ships the Gang looses yet another group of Outcasts. Snotlout is quick to rest the blame solely on Fishlegs and Meatlug whom he dubs slow and slower. Left depressed by what the others said Fishlegs and Meatlug over eat (some berries and rocks respectively) and get stomach aches. The pair go to see Gobber whose solution is to tickle the pair causing them to vomit what they over ate.
A little while later Gobber has experimentally made the iron Meatlug barfed up into a sword, which he discovers to be both lighter and stronger then normal swords. Word spreading to the rest of the village everyone comes looking to get weapons made of "Gronckle Iron" with Hiccup making some into a new Shield. Supplies quickly run out so Gobber and Fishlegs go get more stone only for Fishlegs to realize that he doesn't know what he fed Meatlug.
Meanwhile the Outcasts arrive underneath the island once more, learning that all the Whispering Deaths have hatched, and that one in particular appears to be larger then the rest causing them to run away in fear. Meanwhile the dragon riders realize that the new patrolling formation is not working, needing someone who can fly lower and slower so that a more detailed search can be done Hiccup goes to get Fishlegs back.
The pair of smiths decide to experimentally feed Meatlug various stones and see which ones result in Gronckle iron: Sand Stone makes glass, while a metallic rock over heats Meatlug forcing her to vomit it before she hurts herself or someone else. Pressured by the crowd Fishlegs feeds Meatlug a combination of all the stone which soon proves to disastrous when she is magnetized, soon attracting nearly ever metallic object in the village.
Running away from the large amount of deadly weapons flying at her Meatlug runs to the cliffs and is chased there by Hiccup and Toothless. Before Fishlegs can warn him Hiccups metal leg attaches him to Meatlugs underbelly resulting in him flying around upside down. Fishlegs chases after them on Toothless and together they spot the Outcasts, who decide to kill Toothless and make him into a trophy mount.
Making the most out of a bad situation Hiccup maneuvers Meatlug into a flyby of the Outcast ships and disarms them, follow by expelling the magnetic stone above them sinking their ship with their own Weapons. Fishlegs informs Gobber that he's officially out of the Black Smith business, knowing that his responsibilities to the academy will take up too much of his time, as a parting gift Gobber gives Fishlegs his Gronckle Iron sword.
How the sandstone became glass is a reference to how beaches struck by lightning sometimes become glass due to the silica in the sand.