I have been studying English for 10 years and I'm telling you that saying "a jam" is COMPLETELY wrong!!! You can't say one jam, two jams, three jams and that means that the word "jam" is an UNCOUNTABLE noun. Uncountable nouns don't take a/an in front of them.
I see what pgmbuddy is saying. For instance, I could reasonably say, "Strawberry jam is a jam." The fact that it's a type of jam is implied, but it doesn't need to be specifically stated, similar to how you can say, "The hammer is a tool."