This little piece of jewellery lies underneath the frozen desecrates of Areat. Go to Act III and begin the last quest, waypoint to Bridge of Korsikk and enter the Fields of Slaughter. In there, search for the Caverns of Frost and start boring through monsters till you stagger into the boss mob which is Chiltara that plunges the gibbering gem. Only some people on the community boards signify that the cave most often spawns top left, bottom left and top right of the map and there is frequently just one cave spawning during a single load. If you get Icefall Caves or whatever thing that is not Caverns of Frost, it’s reload time.