Donna's Ring(s)

1. Runaway Bride - plain biodamp (not the same as in Journey's End)
   
2. Partners in Crime - no
3. Fires of Pomepeii - no
4. Planet of the Ood - she's wearing a band on the usual finger (right hand middle)... the biodamp ring again?
   
5. The Sontaran Stratagem - on left hand while she's flying the Tardis (the hand towards the camera), but it's not the same ring as in Journey's End
   
6. The Poison Sky - black ring again during "Back of the neck" on left hand finger
7. The Doctor's Daughter - First appearance!  It's on her right hand while using stethoscope.
8. The Unicorn and the Wasp - nope
9. Silence in the Library - yes
10. Forest of the Dead - flashes of it in some scenes i think, but no clear shot
11. Midnight - can only see one hand:  no
12. Turn Left -  no
13. The Stolen Earth - while at the Shadow Proclamation
14. Journey's End - still wearing it in the Tardis (particularly when the door is slammed on her)

So Donna is wearing the no ring whatsoever in 5-6 episodes, a simple band in "Runaway Bride" and "Planet of the Ood", a black plastic ring in "the Sontaran Strategem" and "the Poison Sky", and the Ring itself in 5-6 episodes, most notably while inadvertently creating the Doctor's clone.

ETA:  Donna is not wearing an engagement ring in "the Runaway Bride"!  She isn't wearing a ring until the Doctor gives her the biodamp ring.  She is wearing a ring in the previews for "The End of Time" which is very different than the grey/black one from "Journey's End."

The Runaway Bride "With this ring I do thee biodamp."	The Planet of the Ood The biodamp ring again?
The Sontaran Strategem The ring Donna wore when she piloted the Tardis.	Journey's End The ring Donna wore during the metacrisis.
The End of Time preview Not the same ring!

At this point, i'm exhausted and grasping at straws. Some people seem to think that Donna becoming a Time Lord would cheapen her experience and growth; i know that erasing her memory cheapens her experience and negates her growth. i'm just trying to find a silver lining in all of this!