Arachne: On The Road to the Valley of Blue Diamonds - Frederick Graves (Ghosts and Goblins 1 1938)
Crane recounts the terrible story of his latest trip to South America, country not specified. He and Manders, with two native guides, go up the river Antoxyl (sounds like an antibiotic) as far as the borders of Peru, in search of a mysterious valley of blue diamonds that is said to be up in the mountains. At a certain point, the terrified guides leave the Englishmen to find it on their own. Naturally they should have stayed far away and minded their own businesses. Reasonably horrible for only 6 pages.