Why is agate called "agate"?

A guy in the office asked that tonight and I'm damned if I know. Thoughts?

 

We had a guy who used to call it "spagat." As in sports agate.

 

I believe it defines one inch of column, or is it limited purely to size of the text used?

 

Always thought it was short for aggregate.

 

It's called agate because it's called agate, roll the bones.

 

"Agate" is a unit of measurement. Typographical, 5.5 points. Two lines per pica, or close to it. It's the smallest type that is still considered readable.
In print advertising, it's the smallest measurement.

 

Indeed, it's the size of type.

 

Yes. I have a large metal ruler from an old NYC ad agency -- so old the telephone number starts with two letters -- and one side of it measures agate lines.

 

I thought it was named after the small rock.

 

I skimmed the Internet, Dooley, and I think it's both. Long ago, the type size now known as "agate" was the smallest that could be readably carved into a signet ring - the kind you'd use to seal a document with wax. The stone most often used for these rings was agate.

 

So both a floor wax and a dessert topping