Chalcedony

Chalcedony is the umbrella term for all microcrystalline (crypto-crystalline) quartz - quartz composed of crystals too small to see individually without a microscope. It's not a single stone but an entire family that includes agate, carnelian, chrysoprase, bloodstone, jasper, onyx, and many others. Understanding chalcedony is understanding the foundation that connects dozens of popular gemstones.

Chalcedony forms when silica-rich fluids deposit microscopic quartz fibers in cavities, fractures, and porous rocks. The fibers are too small to develop visible crystal faces, so chalcedony has a smooth, waxy appearance rather than the glassy faces of macrocrystalline quartz. The deposition happens at relatively low temperatures (below 200 degrees Celsius) from groundwater, making chalcedony one of the most common and widespread gem materials on Earth.

When chalcedony is banded, it's called agate. When it's red-orange, carnelian. When it's green from nickel, chrysoprase. When it's black, onyx. The base material - plain, unbanded chalcedony - is typically pale blue-gray, translucent, and waxy, and is sold under its own name as a distinct gem variety.

Chalcedony in its plain form is identified by its translucent, waxy-to-vitreous luster, pale blue-gray to white color, and hardness of 7. It lacks the visible crystal faces of macrocrystalline quartz and the banding of agate.

Distinguish from opal (softer at 5.5-6, often shows play of color), glass (may have bubbles, different feel), and moonstone (feldspar, different optical effects). Chalcedony's combination of translucency, waxy luster, and hardness 7 is diagnostic.