玉髓是隐晶型的石英，组成十分优良的矿物石英及摩根石。玉髓蜡质光泽，并也有半透明的品种。它的颜色通常是白色或灰色，灰蓝色或树荫下棕色等，从苍白到接近黑色。其他玉髓色调被赋予特定的名字：一个清晰的红玉髓称为'carnelian'或 'sard'。绿色品种由氧化镍(II)，是所谓的绿玛瑙。 红玛瑙有扁平黑白阶。火石也是各种各样的玉髓。

中亚沿江居民的贸易路线都有利用各种形式的玉髓来瓜分环盖和珠。罚款的玉髓物体例子可能是从第一世纪的贵霜帝国开始的发，近年来在阿富汗的西北部发现了许多贵霜印章。

Chalcedony /kælˈsɛdəni/ is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical structure of quartz) is SiO₂ (silicon dioxide).