This puzzle is to be found in the King's Court Hub at the altar inside The Shoppe, you will get the message:
"And it is foretold that by seeking out sand, glass, amd grindstone, one may cross the Crystal Barrier and join with the Master of Hunger. your quest begins at the Fireplace by the forge."
In the Stables you will find a water trough, walk over to it and get the message:
"You possess the pile of sand"
When you have the sand go to the forge and walk up to the furnace and get the message:
"You have melted the sand"
Pick up the glass bead produced and get the message:
"The bead of glass is yours"
The glass bead needs to be turned into a Lens, this you can do at the grindstone which at the Inner Courtyard. If successful you will get the message:
"You have made a Lens of Seeing"
pick it up and:
"You possess the Lens of Seeing"
The Lens of Seeing can be used to cross the Crystal barrier which you will be able to now see through in the Palace Room; when you go through the white archway you will get the message:
"You have been given the power to destroy the Crystal Golem"
Job done, go find the Crystal golem in the Crystal Room.