What is the a button in diamond?

There's no such thing as an "a button" in a diamond. It seems like you might be thinking of something else. Perhaps you are referring to:

* A "culet": This is the small, flat facet at the bottom point of a diamond. It's often referred to as the "button."

* A "girdle": This is the edge of the diamond where the top and bottom portions meet. It can have various shapes and finishes, and sometimes might be described as "buttoned."

* A specific diamond cut: Some diamond cuts, like the "round brilliant", have a particular layout of facets that might make one think of "buttons."

Could you please clarify what you meant by "a button in diamond?" If you can provide more context, I might be able to understand your question better.