The record icon is a circular shape which is usually emphasized with a red color.
Originally it wasn't an icon as such, but rather just a button whose shape was circular. The circular form was probably chosen to clearly stand out from the other angular playback icons. The red color can be a warning to indicate that the function overwrites existing data, or then it is a reference to the red warning light that has commonly been used in recording studios. The red color has also sometimes been used alone on recording buttons without the circular form.
Prior to the circular icon the recording function has also had a symbol which consisted of three wave lines in the ReVox reel-to-reel recorders in the 1950's. These recorders also featured interesting variations of the fast forward, rewind and play icons.
1960's tape recorders such as the Philips Norelco cassette tape recorder.