As somebody who hates white boards and adore black boards (they come in green nowadays actually), chalk is to me something that is synonymous to teaching, and the reason why people in the public transports think I am a baker (or maybe a painter, when I really have lots of it in my pockets and on my jeans...)
First and foremost, whatever you do in your life, you should avoid Stephens.
A much better brand is Crayola.
Above Crayola you have Giotto Robercolor.
This said, you might want to full perfection. Introducing Japan based Hagoromo Fulltouch Chalk, which is known in the mathematics community as the Rolls Royce of chalk.