So far, coal is renewable. Iron and gold ingots are renewable. Redstone is renewable. However, diamonds are not renewable.
So I suggest that diamonds be obtainable from villagers for a lot of emeralds. Villagers that offers diamonds are uncommon/rare. When they do offer diamonds, the amount of Emeralds can be in 12-18. Why is it expensive? To make sure diamonds are not overpowered. Emeralds are renewable. If you can save up emeralds, you can trade them for diamonds!