Spar Mine is a building ideal for getting free Magic Spar.
Depending on how many Spar Mines you built, the cost increases. It's only available when you reached level 12.
1st built = 99 mojos
2nd built = 399 mojos
Also, if you built a portal then you can access your fort, meaning that if your fort reached level 12 you can also spend 99 mojos for another Spar Mine in your fort, thus granting least 2 free magic spars. (3 or more if you got the right conditions.) Be noted that one Spar Mine produces 1 magic spar only per day.