Your schefflera is likely just adjusting to its new environment.  The nurseries "push" them to get maximum growth by providing the brightest sunlight they will take and lots of water and fertilizer.  The shock of being loaded on a truck in the dark, being in a retail store and then into a house with much lower light than in the nursery will cause leaf drop as the plant adjusts.

If the "stones" are soft and jelly-like, they are pellets of polymers that hold water that are designed to give the plant more shelf life.  If they crack open it's probably old pellets of slow-release fertilizer.  If there is just the shell, the fertilizer has already been released.  

Keep the plant evenly moist but not saturated by watering when the top inch of the soil feels dry to the touch.  Water thoroughly so some of the water drains from the bottom of the container.  Don't add more fertilizer right now.  Just allow the plant to adjust to its new home.  

You should see the leaf drop slow down and then stop as the process plays out.  If the plant is pot bound, once the danger of frost is past, repot with new soil and a larger container.  

Good luck with it.

Thank so much for such a detailed answer!!!
I'll follow your instructions and hope my schefflera will be ok soon!

You're welcome. I think your plant will be just fine.