Haven't seen a comprehensive list of these yet and it was something I often wondered and searched for when I first started DA as I didn't want to waste my "precious" moonbeam gems. So for anyone still wondering here is a (not amazingly detailed) list of what the moonbeam gem doors hold. Please forgive my vague instructions and my lack of hard stats, I'm doing these by memory from work.
3 bottom floor doors dug into the cliff face below where Barroch is first introduced. Facing the doors from left to right the room yield as follows.
Left - Trophy Jacket - wearable by all archer capable vocations. Solid defensive stats. Extra ability shortens duration of debilitating effects.
Right - Dragon's Glaze - Longbow- Ice enchanted - Does both physical and magical damage. Approx 400 physical and 200 magic base stats.
Right - Adepts Robe - Wearable by Magick Archer, Mage, and Sorcerer. (Possibly by Mystic Knight but I can't remember for sure) - Good but not great def. Better for Magic Defense. - Extra ability gives each party member in close proximity (but not the wearer) a regeneration effect. Even heals hit points that would normally require curative items. Multiple robes in the same party do stack their effects.
2 other bottom floor doors - accessed through a normal door which opens into a small room which houses the two moonbeam doors.
Left door - Dragon's Ire - Shortbow - Fire enchanted - Like the ranger bow it does physical and magical damage. Starts at close to 400 physical and a little over 200 magic dmg.
Right door - Ring of Perserverance - Increases the discipline points earned by the wearer.
The last three doors are reached by climbing the rocks that allow you to ascend to the top of the broken terrace opposite the entrance terrace (to the right of the door that requires the void key). About halfway up the rocks look for a narrow ledge to the right you can jump on and run to the end of. At the end there is a small nook with a moonbeam door.
Ledge door - Dragon's Roost - Fighter/Assassin Shield - lightning enchanted - Does both physical and magical dmg. Approx 170 physical and 320 magical dmg base.
Climb the rest of the way up the rocks to the far terrace and go to the far end of it near the waterfall. There will be an obvious path you can jump up to enter a small cave with one moonbeam door at the end. Near that same end of the terrace halfway down the stone wall at its edge will be a small passage you can leap to from the broken pillars below. This will house the last moonbeam door.
Upper waterfall adjacent door - Adept's cloak - Very run of the mill cloak with mediocre stats (9 phy def - 5 mag def if memory serves) Has no stated special ability and I have not tested it to find out.
Lower waterfall adjacent door - Ring of Dessication (the real name escapes me) - A ring that prevents the user from being drenched and all problems associated with that (ie lanterns being put out)
So there it is. Sorry it's so vague about some things but hopefully I've given enough info to let people make semi-educated decisions about where to spend their gems.