Follow Up to Earth, Flat, Hook, Sharp

In an earlier post I posed the following question: Earth, flat, hook, sharp. Which does not belong and why? As promised I am posting a follow up which contains my own answer to the riddle which I now believe to be wrong.

My theory was that Earth didn't belong because flat, hook, and sharp are all words that could be used to described the appearance or shape of an object. Take a look at the comments that follow the original post to find what I believe to be the correct answer as provided by my sister Hilary.