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An interdimensional mirror is, as the name indicates, a mirror used by Star Butterfly and other characters to communicate with people from other dimensions.
It operates in much the same way as a cellular telephone: listening to voice commands to make contact, recording and playing messages, and even operating on a minutes-based service whose bill must be paid.
The mirror has a reflective glass in its center, which is the part where the person contacted appears. Around the glass, there is a metallic eight-pointed, star-shaped bezel with orange jewel insets. Though the mirror is mostly operated by voice commands, one of the jewels can be used as a "button" to activate the mirror manually.
In "Monster Arm", Star's mirror is outlined with hearts instead of jewels.
"Blood Moon Ball" shows that Star's mirror can convert into a screen for her to change behind.
The mirrors seem to have a physical connection between each other. In "Storm the Castle", Glossaryck put his hand through Star's mirror, and it came out through the mirror King and Queen Butterfly were using (although that may just have been due to Glossaryck's own magical ability).
In season two, smaller versions of the mirrors, similar in appearance to compact mirrors and acting like cellphones, regularly appear.