It's a Gem. It's a bit rarer than the Jade.
These are all 10 of the different Gems from most common (up to perhaps Level 4 bushes for the Agate) to most rare. I still after about 600 days and over a million bushed whacked never found a Diamond and only ever 2 Emeralds.
Agate, Aquamarine, Citrine, Topaz, Sapphire, Ruby, Jade,Moonstone, Emerald, Diamond

If you want one Moonstone, then you can buy a museum Map for some real cash in the form of BB's from the Haven Museum to get one. To do that Large Map might take a total of about 6,000 Energy used.
You might get one or more from some Rosa Quests in whatever area that you are in eventually to be used open a gate.
That's it other than winning one from a bush whack. A Gem increasing Sword 'might' help you a tiny bit, but those have never helped me at all.
The same can be said for one of the Spells, that might help to increase the odds of you getting slighter higher valued Gems.