With patch 2.5 Blizzard introduced a new unique grand charm type. These charms can break immunes. Lower Res and Conviction still have a 20% efficiency when using these against broken immunes. However, the cold Sorc mastery does not have this penalty making them insanely powerful.
The way to get these charms is to beat Baal in Hell. Once you do that you unlock the new Terror Zones. These zones scale up to match closely to your level increasing exp and the drops. The charms drop from Champion, elite, or unique bosses.
The best zone I have found to find these in is Tristram. It is different than a Terror Stony Field, but the map layout lets you reset quickly, and you can spam these maps very quickly. Kills all the special mobs and reset. Don't bother with the regular packs if you only want these charms. Since the zone changes every hour it's hard to get this specific zone, but if it's up you should be spamming it nonstop.
You can find them in any hell Terror Zone but Trist seems to be the best by far. I've had them drop in other maps but the zone is so spread out that it's hard to collect all the special packs quickly enough.