I do not think medieval civilization is an easy expectation. The medieval town is based on agriculture and large groups of people coming together to make permanent settlements. But... Metal and Gems are non-renewable resources. This means the more of them that get together in one place, the faster they will destroy their food source forcing them to abandon their colony. Instead, they are more likely to form a tribal society with temporary domiciles and small family units in mind.
But, even nomads can have medieval technology; so, instead of towns, they probably live in medieval wagon caravans. They have small forges, plenty of tools, and other things they pack up into their wagons when they are on the go. Their "buildings" are mostly tents, collapsible structures, or simple hovels that they can build quickly. The plateau would need either coal or wood to run their forges to achieve medieval tech as well. Since what you described makes wood sound rare, I'd expect coal to be a major resource, and most of their stuff be made of metal.
As for religion, they would not experience "renewal" the way that we do where every spring we see new food, and new life all around us. This would probably give them a very temporary attitude towards nature where they believe all things come to and end, and when something is gone it is gone forever. This could lead to a stronger bias towards Atheism or Deism without an afterlife. If they believe in Gods, they would probably believe in a Ragnarok like ending to the world.
This does not mean they would necessarily be unhappy and melancholy though. For the most part, their lives would be long and easy until the distant future when metal and gems are no longer so commonplace. Since they don't need to farm or hunt, they'd have lots of time for art, entertainment, and forming strong family bonds... or they could just spend most of their time sitting around like rocks watching the world go by.