Beelzebub

It has become known that the ultimate purple Crystal that existed in the paradise times, is real. Some want it for money, others for power, and there are some who want to destroy it. The treasure is placed inside lighthouse on an isolated island under triple top security measures, but it's still stolen by someone's invisible hand. But main character does not believe in magic. There must be an absolutely logical explanation to the trick and culprit's identity. You just need to figure out, which one.