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What delivery, lol, it's a magical world. I trapped a mage in a mine, and now a pushy teleporter delivers carts of ore to the capital.
Look at the crystals from a different angle - 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, so you have a practically endless supply of these crystals.