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.
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).
There are three different kinds of stones:
Astral Lapis Lazuli - 100 pieces needed
Astral Chrysolite - 80 pieces needed
Astral Topaz - 60 pieces needed
Astral Gems Locations
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.