Require password entry when gems are being spent. The other day my I put my phone in my pocket and when I pulled it out later I had a new draft deck for a premiere draft I had not intentionally bought. My gem account balance was significantly lower. I rarely play draft and I have like 7 or 8 draft tokens stored up so it would have been unlikely for me to use gems to buy in to a draft. Because gems cost money and have value like draft tokens I think it should be much harder to spend them than for a couple in pocket clicks to spend them. Or perhaps a catch like sequence or drag your finger in a design displayed on screen to prove someone really is trying to buy.