What was the twelfth foundation stone of the New Jerusalem's wall?

Answer: Amethyst

The first foundation was jasper, the ninth topaz, and the twelfth amethyst.

“the fifth sardonyx, the sixth carnelian, the seventh chrysolite, the eighth beryl, the ninth topaz, the tenth chrysoprase, the eleventh jacinth, and the twelfth amethyst.”— Revelation 21:20 (BSB)