I do not know if I'm right or not,
the issue is that we should not continue selling and the gems, because at the time of using them so that the mines do not expire it will cost us more coins,

At the beginning it is good to sell gems and get coins, but if you realize they are losing coins because when you use them so that the mines do not expire, you will spend more coins,
that's why they stop using the sale of gems that what they do is raise their value
  and cost more to not expire mines was understood,

If I'm wrong, can you help me understand,