You cannot purchase Dark Crystals directly, however you can earn them through the following activities:
- Fulfilling balloon shipments earns you Dark Crystals
- Selling certain items to the House Decor shop (sometimes the shop pays double for items sold to the shop). You can sell items by tapping them in your warehouse & pressing "Sell to Shop".
- Finishing your League in the Top 3 positions earns you Dark Crystals
- Completing Guest Huts' bonus items earns you Dark Crystals
- Collecting rewards from the Volcanic Portal gives you Dark Crystals (and you get more if you rank in higher leagues)
- Removing weeds sometimes gives you Dark Crystals