Hi Katie:
You have discovered that English words can be different parts of speech.
Stone can be a noun: The ceiling was of stone. Stone from the quarries was used to build the pyramids. Her ring had a semi-precious stone. The boys threw stones at the window and broke it.
Stone can be an adjective: This old stone wall is covered with moss.
SIlk is the same - it can be a noun or an adjective.