Astral Gemstones are quest items needed for quest (Archetype)'s Path given by archetype trainer at level 40. The reward for completing this quest is additional ruby. Player needs to have Astral Illumination buff (ask the trainer to illuminate you) to see the stones. Illumination needs to be regained after the death of the character.
Spawn points of the gems are in all zones for levels 10+ and when character loots it she can get either the gem or Astral Gemstone Shard (vendor trash).
Spawn points for Astral Gems work very similar to Mystery Stones known from 'Antropologists' quests'. Every zone has several spawn circles and gems spawn on several spawning points within these circles. Player can see only one Astral Gem spawn in each circle. When she loots it, the gem spawns almost instantly on different place within that circle and is visible only for other players. Respawn time per circle is individual for each player - approximately 30 minutes.
Maps below show the spawn points, not the circles.