While playing as an Empire, I was amazed to discover a Crystal Crag. I promptly built a town next to it and researched Arcane Experiments. After researching Arcane Experiments, I still couldn't build a mine on it. The text says I need to research Arcane Weapons in order to harvest, but Arcane Weapons is now the last tech in the crystal tech line (after Arcane Armor now). I suspect that when these techs got switched around, the tech to unlock the crystal mine didn't get changed as well. Since Arcane Experiments adds items that use crystals, it seems the logical tech to unlock crystal mines.