Hi, here is a brief guide on the Z-Crystal ^-^

What is a Z-Crystal?
A Z-Crystal is an item required to upgrade moves to Z-Moves.

Z-Crystal for each type:
1. Buginium
2. Darkinium
3. Dragonium
4. Electrium
5. Fairium
6. Fightinium
7. Firium
8. Flyinium
9. Ghostium
10. Grassium
11. Groundium
12. Icium
13. Normalium
14. Poisonium
15. Psychium
16. Rockium
17. Steelium
18. Waterium

You can craft them with a gem surrounded by crystal blocks. (You can buy them for 64 battle points in the battle shop).

Then there are Z-Crystal specific Pokémon:
1. Aloraichium
2. Decidium
3. Eevium
4. Incinium
5. Kommonium
6. Lunalium
7. Lycanium
8. Marshadium
9. Mewnium
10. Mimikium
11. Pikanium
12. Pikashunium
13. Primarium
14. Solganium
15. Snorlium
16. Tapunium
17. Ultranecrozium

They are not craftable, but you can get them from boss. (You can buy them for 100 battle points in the battle shop).

I hope I have been helpful! :-d