Chalk is uncountable. "The new teacher is out of chalk." "I need some chalk." "Do you like to use chalk." A stick of chalk or a piece of chalk is countable. "There are ten sticks of chalk in the box." "I have only one stick of chalk left." This is like coffee and tea. When I state preferences ... "I like coffee and he likes tea." (Uncountable) When I order in a restaurant ... "I'd like a cup of coffee and he'd like a cup of tea." (Countable) But when speaking informally, I shorten it, drop some words, and say, "I'd like a coffee and he'd like a tea." I mean a cup of coffee (countable) but it sounds like I broke a grammar rule. I have never heard "chalk" used this way. Fadwa, are speakers near you using it this way?

Technically it can be both. Chalk (uncountable)- The material or substance chalk. Chalk (countable)- A piece or stick of chalk. If you said "chalk" or "a chalk" I think a native speaker would easily assume either of those definitions. Calling it a stick or a piece instead would be preferred, but I have heard educated native speakers saying just "chalk" like that. Maybe it's an informal shorthand. But chalk is probably a special case, because maybe one of the most frequently seen uses of chalk is in the form of chalk sticks, like in school). If you tried to apply this rule to any substance, without a proper quantifier (drop, piece, etc.), there is a high chance it'll sound super weird and just wrong. Like gasoline. It's uncountable. You can't say "a gasoline". But "a chalk"? Semantically, no. But technically, yes.

Uncountable. You would indicate the amount of chalk you need by using the appropriate determiner. For example, can I have a piece of chalk. Here the letter 'a' tells you it is one piece the person wants.