And then after you've exhausted your explore horses for the day, use up your player energy in the mine. https://horsefable.com/dark-horse-mine The newer gems (amethyst, citrine, onyx and rose quartz) can earn you more but the older gems are pretty saturated in the market so I find those only get 2-3k. I'm willing to pay 25k for amethyst.
You can also create another account if you're a real tight spot for money if you don't have any already. You can have up to three accounts. You do have to use different emails for each account though. And there's no rules about not being able to share funds/items between accounts.