You can get it only in October.
A bone and an invisible dragon
You can get in only in October. You can breed a Bone Dragon by a Fire and Earth Dragon. I have 2 Bone Dragons by that combination. You can buy a Bone Dragon for 25 gems.
You can't breed a Bone Dragon after Halloween. To breed a Bone Dragon: Fire and Earth Dragon (level 10)
The bone dragon is the best dragon to race in the Boneyard Chaldea.
You breed fire and earth dragons though sometimes you might get a lava dragon instead!!
There is no such thing as a Birthstone Dragon on DragonVale
The bone dragon is available around Halloween. Different combinations of the Earth Dragon and Fire Dragon are the most effective ways of getting a Bone Dragon. Fire (level 10 left) and Earth (level 10 right) seems to be the most effective method.
Possible results of fire + obsidian:LavaObsidianEmeraldCactusMotleyTreeAirSparkPoisonAbyssDouble Rainbow Dragon
As of July 2012, there has not been a star dragon in DragonVale.
You don't breed a Kairos Dragon in DragonVale. You have to get parts to complete it.
To date, there is no grass dragon in DragonVale. You may be thinking of the plant dragon. See the related question.
The incubation time for a rainbow dragon in DragonVale is 48 hours.