I think these are all possibilities. Ruby is definitely too strong. You got so much value, dealing 5 damage to a minion, having a strong body, and gaining armor. Mithril I feel is not good enough, you have to have a big board and compared to ruby, it is bad. Onyx is okay but I feel it won’t be used in rogue. Deathrattle rogue maybe, but the battlecry effect would rarely be used. I feel the Emerald one doesn’t really fit the Druid class, but it’s not bad. I think sapphire might be a bit too good, maybe it would be more balanced as a 4/6.