Can't delete boxes opened with gems??

If you click the gem-opened box, the sale price appears at the bottom. Sell them to earn coins. I also gather fruits and sell them off. Sometimes merging multiple fruits yields one with a higher coin value, but it's actually more profitable to just sell them without merging.

If you click on an item, it will appear at the bottom. I suspect you might not have found it because the triangle menu at the bottom was folded. If you expand the triangle menu, the price will appear as shown in the picture.

I sold all the chests that open with gems. The coins accumulate surprisingly well.

Jewelry boxes give at least 20 coins, making them a useful source of income. You might also consider building a coin warehouse to increase your coin reserves.