Mystery Block/Item

I got this idea when I woke up, seemed quite random but could be interesting; add a new block with a mysterious texture, that is hard to craft/find, and later, in the next update, it is replaced with something else. It could be useful, or completely useless, but it would make players have to decide if they think it'll be worth making the thing before they know if it's actually going to be useful..

Maybe it doesn't make sense, but I wanted to write this here anyway... lol