[Spoilers]

I've been keeping a list of the locations mostly so I have something to cross-reference with if I ever decide to collect them all via some list online. It's not worded 100% clearly, but here are the 20 I have so far. If nothing in the description is familiar to you, you might not have found it yet.

1 Abandoned tower after tutorial, near trap and troll 2 Old Wreck, west of Old Dam Lookout 3 Beach with a King Crab on peninsula south of Old Wreck, following coastline to the east of Old Three River Watch 4 Burned Off Fishing Lodge, bay close to Yero's house 5 Old Sherath, locked lake south of Old Three River Watch 6 Basement of Golden Ford Castle 7 East of Deserted Ruin, Northwest of Riverville, by an overturned cart 8 Miner cabin, mountain pass to Ark, west of Riverville 9 Abandoned customs facility, path to Ark down from mountain pass by the crypt 10 Under the bridge to Ark 11 Near the sun temple in Ark, past the gardener and down the path by a tower 12 Top of lighthouse in harbour south of Ark 13 Basement of the harbour in the freezer 14 Ark Barracks District, near entrance to Marketplace, within the walls through a small archway 15 Old Camp in Undercity 16 Waterfall at start of river to the northeast of Sun Temple, at the treasure spot indicated by the map you may choose as reward for the quest Wave Taming 17 In main quest at Old Dothulgrad, after cutscene with Calia, follow path she came from 18 Aged Man's manor in Western Cliffs, in side room to the right of entrance 19 After speaking with the Aged Man, in the corner of the courtyard 20 At the top of Enderal's eastern snowy mountain, to the Northwest of Old Uskasarak, past the fourtusks (mammoth) and frost troll, in a crevice close to a corundrum deposit.

Feel free to add to this.