What would you call a person who can turn some random weird-ass weed into a load of cash? A meth cook? yes, but we are in a fantasy world, so please don't bring them up here. No, I am talking about the alchemists. Those potion makers are akin to a golden goose, you feed / give them grass then they will lay golden eggs / coins for you. They are so valued by almost everyone, to the point if a war between human broke out, you will not see no one lay a finger on them. That's a privilege only Healers can match.
But what is it exactly that make the alchemist so highly valued? Is it because they can make so much money? But if it is only about wealth, big merchants can make them too, yet they are still get targeted by robber in time of peace, and plunderer in time of war. Well, the answer is the potion the alchemist created.
You see, among many things, there are two products that were so sought after from them: Mana stone and Mana-free water. Each is connected to another.
As you recall, wild water -as people called it here- is contaminated by ambient mana. Be it water from the river, the well, the rain, the dew, or from deep underground, it always contains mana, which makes them undrinkable. But human need to drink more than they need to eat, so humanity get a big problem here, and that's where the alchemists come.
From an alchemist I know, there is this substance called Macus, which able to separate and crystalize the mana from the water. They need only make the substance, mix it to the wild water, boil them up for few minutes to speed up the reaction, let it cool down for few hours, then voila, they've purified the wild water and created two things: Mana crystal / stone as the main product, and Mana-free water as the byproduct, both are sellable.
The key substance, Macus, is made from various kind of weird-looking roots, grass, and flowers. All of them can be found at either the outskirt or the deeper parts of the woods. Gathering those ingredients is what makes the herb gathering quests always available on the quest board in the guild. Along with hunting quests commissioned by butcher shops, the two are constant source of adventurer's work and income. As long as humanity exist, people will always need meat to eat and water to drink.
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No, creating Macus is not easy even if you have all the necessary item and step-by-step instruction to make them. It requires a steady hand, keen eyes, and a lot of patience. As the saying goes, everybody wants gold, but nobody wants to be gold miner.
Now that you have understand that alchemist is one of critical element that humanity needs to survive, another question might have popped up in your mind: What mana stone is used for, and why drinking water is still as expensive as decade-old wine (At least for people who can't use magic) if it can be purified at large scale?
Well, mana stone is mainly used as a fuel to erect the barrier that protecting the city. Of course, the exact amount of mana stone needed is depend on how large the city itself, but as you and I can imagine, the minimum amount probably still fall in 'A LOT' category. Besides to fuel the barrier, mana stone is also used to fuel various kind of luxurious magic item created by the enchanters, like portable person-size barrier and many more.
As to why drinking water is still ends up as expensive as decade-old wine for people who can't conjure their own water, the answer lay in a detail I haven't mention yet. You see, the mana-free water that was created as byproduct in mana stone manufacturing process is indeed, free of ambient mana and safe to drink. BUT, there is one basic thing about the water that have been altered: Its taste.
Yes, the mana-free water somehow is no longer tasteless anymore, it taste like rinsed water, rendering it back to be undrinkable, even though health-wise it is fine. So here is the trick the alchemists employed, they sell the water to the breweries. The water will be used as ingredient and then turned into somewhat tasty liquor. That's how everyone get access to ale and its equivalent in the bar and the inn, but not to normal drinking water. Other than that, the alchemists also sell it as general water to everyone who wanted it. It can be used to wash clothes or rinse the dishes. Either way, the alchemists sell the water at very low price: two gallons / copper, because it is only byproduct anyway.
With that, the alchemist can get paid for getting rid of their waste, and everyone now can get a drink, albeit an alcoholic one. That's why there is no alchemist who went astray and become a meth cook, they are already rich by cooking a different kind of crystal altogether. That's also how the alchemist become so highly valued. They, along with the Enchanters, the Healers, and the Archmages are vital to sustain and protect human's life.