Today, I heard the news that a piece of ruby was stolen in Tokyo.

"piece of ruby" sounds like only a part of a ruby was stolen, when I think the whole thing went missing. "a ruby" also sounds a little more natural.

The ruby iweighs four kilograms, and its price is three billion yen (about 30 million dollars).

"weighs" instead of "is" sounds a little better

However, there is additional information that the police say the ruby wasn't stolen, and. ¶
Instead, a woman who owns the ruby was in negotiation, and she brought it back.

I think the original sentence is really long, so splitting it into two sentences flows better and makes the punctuation easier. Alternatively, I might write: "However, there is additional information that the police say the ruby wasn't stolen; a woman who owns the ruby was in negotiation, and she brought it back."