I was about to say Lapis Lazuli as well. For how rare the ore is you would expect it to have a major purpose. But I dont really use it for anything except making 9x blocks out of it and perhaps throwing it into a house wall for some decorative purpose.
Someone above was about to say Clay? You make bricks out of clay! Best house building item .... that is not useless! ...
I never really thought about it, but I remember I saw bats in the hull of a ship on Minecraft and I was like, "What the heck are those things?" but then I realized they were bats after a few seconds... I think they're pretty pointless.
Silverfish are not formidable in any way, but they can pose a big threat when mining in ravines. I've died many times while mining on the upper levels of a ravine, thanks to a silverfish bursting out of the block in front of me and knocking me off the ledge.