That lacks originality man .. think outside of the box!

Though I do like the cave image, the farms could be located in some damp caverns crawling with plantlife and bugs. There would be pools of water and crystals randomly popping up on the walls, floor and ceiling.
Some of those crystals could transform into glyphs then when they are treated in the right way.
And if there will be a quest to show someone how to make glyphs I suggest the quest is very limited so only a handful of people can discover that knowledge. Either because the quest is only available a limited number of times or that the requirements to recieve or complete the quest are so high, not everyone will be able to complete it. For example you \'d need to be a very skilled magician along with having a lot of intelligence to understand the knowledge. One would also need to have a substantial ammount of charisma to convince whoever posseses the knowledge to cultivate glyphs to share it with you. Then one would need to posses knowledge of the brown way for example to be able to grow the glyphs as well as possesing the right type of spells, resources and the proper location.