p.s.
What delivery, lol, this is a magical world. You trap a mage in a mine, and a pusher teleports carts of ore to your capital.
Look at the crystals from a different perspective - you have gold (instead of ore, sulfur, or mercury, which you exchange for gems), you have 2-3 cities with markets, and a hero - a scout with a specialization that gives +1 market. You have practically an endless supply of these crystals. Well, at least as long as you have gold. =)