The diamond is in the Valley of the Lost in the seventh book in the series. It is the last gem that Lief needs to collect to complete the belt. To obtain the diamond, Lief, Barta and Jasmine must face a guardian they did not expect to meet.
The emerald is the fifth stone the heroes are able to obtain. Lief, Barta and Jasmine travel to Dread Mountain where they encounter the guardian of the emerald, the toad Gellick, who controls all of the Dread gnomes that live on the mountain. The toad's spit can turn a man to stone.
The lapis lazuli, a blue sparkling stone, is the fourth stone acquired during the quest. It is found in the book "The Shifting Sands." An unknown guardian that includes a hive of insects guards the gem in a barren wasteland.
The golden topaz is the first gem the characters find in the series. Lief and Barta travel to the Forests of Silence where they meet the wild girl Jasmine who will accompany them for the remainder of the quest. Jasmine helps defeat the guardian of the topaz, the Golden Knight.
After finding two gems, the characters have additional powers and confidence to continue the journey. The trio ventures to the City of Rats to find the third gem, an opal.
The opal's guardian is a snake that wears a crown with the jewel attached.
The ruby is recovered in the second book in the series, "The Lake of Tears." A giant fish maintains the ruby, but it is not the only monster the trio must face in this adventure. A servant of the sorceress Thaegan guards the Lake of Tears and must be defeated for the group to move forward with the quest.
The amethyst is found in the book "The Maze of the Beast." A group of pirates kidnap and rob Lief, Barta and Jasmine, then throw them into the bowels of the earth, which is the Maze of the Beast. The gem is installed high up on a pillar within the underground caves. The heroes must find a way to retrieve the amethyst without being killed by the Beast, a blind slug that can capture them with its spit.