

From last weeks episode of Chiko-chan will scold you, if you are learning Japanese, you might be wondering why Furigana is also referred to as "ruby/ルビー”
It was gained from a Japanese person studying the printing press.
“Ruby” refers to the size of the font, which is at 5.25 mm. Since Furigana is shown as a size of a gem. The term itself is derived from religious books as they are named after jewels with the size of the letters used in the book. 7 refers to Ruby, which is the same size font used at 5.25 pt. They also compared the other sizes referred to as Emerald (6 pt) and Diamond (4.5 pt).
Furigana is basically Ruby text used as a reading guide. Of course with the use of Furigana, it improved reading literacy in Japan. Of course, songs will use a different reading although the Kanji means something different. Furigana is used in this case.