The Soul Stones Shop becomes available once you have evolved at least one of your heroes to the Absolute Star (i.e., six stars) or received Soul Coins for the first time. You can access it by tapping Merchant on the main game screen:
Here you can spend Soul Coins to purchase Artifact Chest Keys, Summoning Spheres, Huge EXP Potions, hero equipment, and Jet's Soul Stones. New goods appear every 24 hours at 9:00 PM your in-game time. You can also refresh the Shop with emeralds.
Note: The Soul Stones Shop is the only place where you can get Jet.
You get Soul Coins by exchanging Soul Stones of absolute evolution heroes. The exchange rate is 1 Soul Stone = 100 Soul Coins. Once you open the shop, the game will offer to trade the excessive Soul Stones. If you do not want to trade the Stones, just press the close button in the upper-right corner of the exchange window.