Fire Amber/Firestone

It was thirty years ago when a greenseekers expedition was returning from the southern parts of Equestria. They were supposed to be met by a hunters patrol that would help them back to the Empire through the snowy mountains, but despite waiting in the meeting place for three days nobody came for them. They were running low on food, so as a desperate decision, the leader of the expedition decided to set off and try to find their way to the Empire alone. But that was a decision they had to pay in blood, for even after weeks of wondering through thee frozen mountains they hadn't managed to get a single glimpse of the Eclipse Tower, and soon enough they started losing members of the expedition.

One by one, they succumbed to the cold of the arctic north, until there were only three of them left. And in their despair, trying to find shelter from the snow storm, they stumbled into a cave. But something was strange with that cave: inside, there was dripping water from the ceiling, while in that cold, there shouldn't be no running water. And it didn't take long for them to notice that there was warmth radiating from deeper in the cave. So they went to investigate, and after walking for a while they reached a big opening in the cave: 

There was a huge room inside the mountain, filled with a warm, amber glow. But this was different than regular amber, for it was actually, warm. Andd the walls were made entirely of this stone. But that wasn't all. Apparently, the warmth of the stones created a perfect enviroment for plantts to grow: it was like nothing they have seen before so far north. It was like one of those jungles they would find in the southern parts of Equestria, completely covered in plants of all kinds.

Quickly, the greenseekers took out a few seeds and plants they had been carrying with them - which were on the point of dying by now. As they put those on the warm ground, they looked as if they were magically revived.

They looked at eachother and didn't say a word, but they all knew what they were thinking: back in the empire, there was absolutely no way to grow plants. But this - this might allow them to do so! And that would do so much for the herbalists guild and the Empire... They HAD to get these firestones back to the Frozen Empire. They couldn't die now.

But how were they gonna do that? They were lost, they could've been miles away from the Empire, and probably nobody would ever find them. But at least they had food now - the cave was full of plants and fruit they could eat. So they were simply going to wait for now.

A few days later, they were found by a hunter patrol that managed to follow their trails, and they took them back to the Empire together with some Firestones. The Herbalists then started experimenting with the stones and managed to start growing plants for the first time in the history of the Empire. However, the stone's warmth seemed to slowly fade away, and 6 month later it was almost gone. Therefore, a new hunter patrol was sent to gather more stones. It seemed like the mountain in which the stones grew had magical properties, but the stones were mere simple amber once taken away from their cave. 

Still, that allowed the Herbalists to grow plants, even if in small quantity. And since then, the Herbalists Guild became a very important guild in the empire, despite still being just an underguild for the Scholars Guild.