[Grammar] plural

[Unregistered]

Sometimes words are used in singular or plural forms and can be hard to understand, could you please clarify?
Many thanks

e.g lumps of rock....why is rock not written as rocks?

 

[emsr2d2]

Sometimes words are used in singular or plural forms and can be hard to understand, could you please clarify?
Many thanks

e.g lumps of rock....why is rock not written as rocks?

Rock here is the generic, collective term for the substance that the lumps are made of.

It's the same with "slices of pizza", "pieces of coal", "handfuls of grass"...... etc.