A solid stone band is the simplest form of ring and the hardest to make well, because the material cannot be sized afterwards — it has to be cut to the right inner diameter from the start. If it is wrong, the stone is scrap. That is why stone bangles and rings are sold in fixed sizes rather than as adjustable pieces, and why we list two sizes here instead of pretending one will fit everyone.
Nephrite is a good material for this because it is genuinely tough — its interlocking fibrous structure resists the chipping that would ruin a quartz band. It is not indestructible: a drop onto tile or a stone floor will break it, and we say so rather than letting you find out.
Grey-green nephrite like this is the common material. If you see a vivid emerald-green stone band at anything near this price, ask what it actually is.