Bēryllos (βήρυλλος) refers to the precious stone beryl — a transparent to translucent gem ranging from green to sea-blue or golden-yellow. In Revelation 21:20, it is listed as the eighth foundation stone of the New Jerusalem. The word is borrowed from Sanskrit through Persian and Semitic languages. Ancient beryls included what we now call emerald and aquamarine.
Beryl in biblical imagery appears in visions of divine glory. The wheels of Ezekiel's chariot-throne (merkabah) gleamed like beryl/chrysolite (Ezekiel 1:16; 10:9). In Revelation 21, beryl's place as the eighth foundation of the New Jerusalem may connect to the symbolism of eight as new beginning (eight people on Noah's ark, circumcision on the eighth day, resurrection on the first day of the new week — the "eighth day"). The New Jerusalem's walls of precious stones declare that God's eternal city is built on character and covenant, not merely material splendor.