No rule for this one
No ending or word group predicts the article of Spiegel. Learn it as a pair and say it out loud: der Spiegel.
der Spiegel
It's der Spiegel. Spiegel is masculine, plural die Spiegel.
Der Spiegel ist im Bad.
The mirror is in the bathroom.
No ending or word group predicts the article of Spiegel. Learn it as a pair and say it out loud: der Spiegel.