On 17 Jan, 14:41, Owain wrote:

From my younger days I seem to remember that play chalk was a lot
'grittier' than teachers' chalk, as well as the sticks being thinner
and breaking more easily.

"School" chalk is usually anti-dust, which is a coating over the
sticks. This has a detrimental effect on fingernails, making them
brittle one reason why teachers had short fingernails.

Nowadays I mostly use french chalk (welding shop), as it draws on hot
steel.