You can collect one random Elemental Stone every 6 hours from the Home Shrine,which is unlocked by default.
Shrines for specific elements (Fire, Water, EarthandAir) give you their respective Elemental Stone and no other. There are 2 of each of these Shrines on the Legend of Tengguan farm. Unlocking 2 of the same time of Shrine will allow you to to collect 2 of that Elemental Stone every 6 hours.
Two more unique Shrines will be available for unlocking on 8th May.
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FarmVille 2: Country Escape Game GUIDE
哈利波特：巫師聯盟