Chalcedony

Chalcedony
Chemical composition	Silicon dioxide SiO2
Crystal system	Trigonal
Habit	Aggregate
Fracture	Conchoidal
Hardness	6.5-7
Refractive index	1.535 - 1.539
Specific gravity	2.60
Lustre	Vitreous

Chalcedony is microcrystalline (cryptocrystalline) quartz. The name is often used to encompass all varieties of microcrystalline quartz such as jasper, carnelian, chrysoprase and bloodstone. The mineral is very abundant and found all over the world. The most common variety is a muted blue-gray tone (unnamed) found in geodes and vugs.