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A few Reckoning Gem locations (missing entries on http://www.ac2hero.com/items/reckoning.php):

Burial Grounds: Drudge Outcast camp (South of Millstone)
Dryreach Mounds: North-East Mad Crone outpost (Rithwic ringway)
Transcendence: near Hero Shrine, North West Esper outpost (Kehan ringway)

Osteth Armory: Around the Osteth Soulbound Weapons Golems (far north of the Southern Prosper Outpost)

The currently listed item "Prosper Overwatch" is actually "Prosper Observatory", and its item level is 23, not 15. So its icon color is the same as the other level 23 Reckoning Gems.

Shoyanen's Tower: East of Mayoi

Tukal Aqueduct: SE of Arhovas, around The Icefall
Lyceum's Shores: NW Nepeth Outpost (1st rw)