The setting is contemporaneous with publication: the beginning of the twentieth century. Crawfurd grows up in Kirkcaple, by the North Sea, where he first encounters the antagonist, Laputa, performing a ritual on the beach. Crawfurd's father dies, and he goes to work as a shopkeeper in a place called Blaauwildebeestefontein.
Crawfurd comes into contact with a Portuguese man, Henriques, and again with Laputa, and he gradually learns of illegal diamond smuggling and of a planned rising of the native people of the region, including the Zulu people and the Swazi people, led by Laputa. Laputa's skill as a preacher allows him to inspire many tribes across the region to follow him, and he invokes the legend of Prester John and positions himself as the rightful heir and leader who can rise up against colonial rule. Crawfurd learns more about this after meeting Captain Arcoll, who leads the colonialist army and police.
Using information learnt from having overheard the conversation of Laputa and Henriques, Crawfurd infiltrates the cave where the tribal leaders are gathering and witnesses Laputa commencing the rising, wearing the necklet of Prester John, which legitimises his leadership. Crawfurd is captured, but having managed to relay a message to Captain Arcoll, escapes during an ambush and steals the necklet from the hands of Henriques, who is trying to steal it for himself. After running all night, Crawfurd is climbing a ravine in the escarpment up to the plateau above the berg when he is captured again. But he manages first to hide the necklet, which is made of priceless rubies.
After being taken to Laputa's new base, Crawfurd escapes immediate punishment by offering Laputa his knowledge of the location of the necklet in exchange for sparing his life. Laputa, who needs the necklet in order to convince his followers, but has not told anyone of its loss, goes alone with Crawfurd to search for the necklet. In the ravine, Crawfurd narrowly escapes once again and steals Laputa's horse to take him to Arcoll's headquarters.
With Laputa separated from his army, Arcoll's forces are able to quell the leaderless uprising. Meanwhile, Crawfurd returns to the cave, where he finds the treacherous Henriques dead outside, having been strangled by Laputa. Entering the cave, Crawfurd meets Laputa, who by now knows that all his plans have failed. Laputa destroys a rock bridge giving access to the cave, and then commits suicide by jumping into an underground river chasm.
Crawfurd makes a daring escape by climbing a cascade up and out of the cave. He rejoins Arcoll and is instrumental in bringing about the disarmament of the native uprising and the subsequent peace. With Arcoll's help he is rewarded with a large portion of the treasure hidden in the cave and eventually returns to Scotland a rich man.

Question: WHAT PRECIOUS GEMS WERE PRESTON JOHN'S NECKLET MADE OF?
Answer: RUBIES
Alternative answer: Rubies.