I see some issues with your currently planned system:
- Flood filter. Well-intentioned, but too harsh, if sensible at all. Once a month? Much too long. If your player wants to refer people, he wants to refer as many as possible (for reward and/or because he genuinely wants other people to enjoy your game), and you want as many players, most preferably paying ones, as possible. So with that spam filter you set a hard limit on what both parties want to achieve. Bad idea.
- A much better way of limiting floods is to improve the quality of invites, i.e. as already suggested, give rewards only if the referred player actually spends money on your game. Second, if you're already planning to track the invites, then communicate and apply an anti-spamming policy, such as manually punishing top spam offenders, for example by removing their invite bonuses temporarily or permanently.
Mail based invites. Certainly works, but why not use social media, like Facebook? Much more efficient and the preferred way of recommending stuff nowadays, and really a no-brainer on smart mobiles, which all your potential players have anyway.
- Switching from Mail to social media also greatly reduces the ability for and attractiveness of spamming, because such sites typically already have anti-spamming policies in place, and at least there is a reputation at stake for the potential offenders.
As for the actual reward, as suggested, give rewards only for referred players that actually pay. That also removes the rationale behind farming because whoever the actual person behind a device/purchase is, you earn money.
Make both players, the referrer and the referral, benefit from the invite and the first purchase. Give them both some free Gems, or exclusive items, or a discount on their next purchase, or on purchases for the next X dollars (or whatever you real life currency is), maybe even a multi-level referral system like League of Legends has, or whatever. Even something as simple as a prominent "top referrers list", measured by the money the referrals spend, and with no other benefits for the referrer, can help boost your referrals, because people very simply like to be on top of lists.
And, for the love of the Gaming Gods, if you make a Trading Card Game, either make the cards tradable or remove the Trading bit.