I've been playing PoR for a long time.

But now, I don't know to proceed the game.

I know that i've to get into the Catacombs, and the House of Gems.

Can anyone tell me how to get there.

SPOILERS BELOW...

Are you familiar with the Glim-Gardens? In the north part of this area, there is a path that leads to the area around the House of Gems. There is a pump house on the House of Gems grounds with a "spawn pool" and a woman's body pinned to it with a sword. Remove the sword, and the pump house starts working again.

Once the pump house is restored, the ghost of Harldain can fully come to life inside the House of Gems. He gives you a stone to complete the Wizard's Torc and tells you how to use it to enter the Elven Catacombs from the amphitheatre in the Speculum Grounds.