He was reading about diamonds when his father entered his room.
(Was he reading a book, or was he reading passage about diamonds on the Internet?)
It could be either one. The word "reading" by itself doesn't tell us WHAT he was reading.
To determine whether he was reading a book or an online passage about diamonds, you can look for contextual clues in the given sentence. However, the sentence itself does not provide enough information to differentiate between the two possibilities. It is possible that he was reading a book about diamonds, or he could have been reading an online passage about diamonds.
To get a definite answer, you would need more specific information about the medium through which he was reading about diamonds. For example, if the sentence included specifics such as "He was engrossed in a book about diamonds" or "He was scrolling through an online article about diamonds," it would provide clear context.