Kundan jewellery is very delicate, and are often intricate in design So, one has to be very gentle while cleaning them:
Use a soft piece of suede cloth to wipe the surface of the jewellery clean.
Take a small piece of common eraser (rubber), and again wipe all surfaces clean using it, to remove stains and marks.
Take a baby brush, dip it in a bit of face powder, and give the piece a brush-over, and then, air it for a while.
Always keep a silica pouch in the box you keep kundan jewellery in. It keeps it from tarnishing and cracking.
elegant neckpiece of Kundan jewellery