Certain websites and individuals might offer discounted Gems/Diamonds. Don't be fooled - it's a scam.
Such practices request private login data (such as Apple ID, Google Play credentials, etc.) in order to access your game account.
Offering Gems cheaper than what their price is inside the games store is only possible by utilising e.g. stolen credit cards or other fraudulent means - do not get involved in this kind of activity.
Purchasing Gems or Diamonds from 3rd party vendors can lead to revoked in-app currency and can even get your account permanently banned.