I feel your pain. I healed the DK tank at Stone Guard hc last night too. Mana problems like hell (I didn't have the darkmoon trinket yet at that point - only got it afterwards). But I felt so inferior to the monk and the druid, it wasn't funny. They both had got the darkmoon trinket while I was at work that day though, so they had it already whereas I didn't. But now I do ^^
We had already had a day of progress on Stone Guard during which I learnt to throw away my mastery reforge again *sniffles sadly* and go back to a full crit reforge and spirit/crit gemming, plus Glyph of Totemic Recall to make HST free and Glyph of Water Shield as that one seems to make some sense in that encounter, and I picked Unleashed Fury to make my heals on the tank larger.
Of course, it didn't help that the tiles weren't lit up nicely on the second round of tiles. It's really important to get at least 90 stacks up, best would be 110, and then pop mana tide on the first and third set of tiles lit. We only had 70 on the second round, and that wrecked healer mana. It was a bit of a consolation to see that the other healers were oom at the end as well, but this encounter feels like I'm battling my mana bar instead of a boss, and I've always prided myself on my ability to manage my mana well
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I kept RT on CD, also trying to throw one to the other tank whenever I could, but for the most part, I was busy with spam-healing the dk tank with mostly Healing Surges. Occasionally, it felt like he wouldn't gain any hp at all - which seems to be a bug sometimes... I don't know. We've had it a few times during MoP raids already that the tank would die and you'd check the logs and they'd be all "Serisse's Riptide +0.0" or "Atalus's Beacon +0.0 while not being at full health. Weird. I think it was always the dk tank when that happened.
Also, I wish to state that Amethyst, Cobalt and Jasper is a crappy combination and I hate it.