In the Forgotten Vale, in Dawnguard, when you defeat Frost Giants, some of them drop XXX Paragons, such as Sapphire, Amethyst, and Diamond. Near one of the giants, at the top of a series of waterfalls, is a small broken ruin, little more than a platform and a broken archway. If you insert the paragon into the receptacle, a magic portal will open. Stepping through, you’ll be in a part of a dungeon you couldn’t otherwise reach with treasures around you, either loose or in chests. I’ve picked up quite a few potions, gems, gold coins, and scrolls from these areas. They are well worth the journey. Also, don’t forget to grab the paragon when you jump back through the portal! They’re worth upwards of 1000 gold themselves!
Better location details: From the Shrine of Learning: head NW up the river. Go up the ancient stairway/hill between two waterfalls. At a ledge, you can turn left, look over the gorge and see another wayshrine. Instead, turn right. You’ll see another frost giant’s lair at the base of a waterfall across the river. Drop down, and the portal is on your side of the river.