How to make disappearing item?

I am creating a gem which is unstable when you mine it. I wanted to slowly damage it while it's in your inventory or in a chest. And damage it even faster if you drop it. And I've tried to deal 1 damage with an if statement of "if seconds past current minute = 5" then "deal 1 damage to provided itemstack"

But it doesn't work. At all. I tried searching this but it doesn't seem to be a discussed topic. So I thought I would make a forum post asking for help. Any advice you guys can give?