Chalcedony is quartz, but very fine crystals, which produces a very dense and durable mineral.

Chemical Formula: SiO2
Class: Silicates, Tectosilicates
Hardness: 7
Coloration: White, purple, yellow, orange, green

How to identify it/Unique Characteristics or Properties: Chalcedony is quartz, but very fine crystals, which produces a very dense and durable mineral. Small inclusions of other minerals also give chalcedony a wide range of colors, most of which have other names for them. Different names include:

Agate: Circularly banded in any color.
Carnelian: Yellow to deep red from hematite, translucent.
Bloodstone/Heliotrope: Green from iron with red specks from hematite.
Chalcedony: If they are only called chalcedony, it is white.
Chrysoprase: Green from hydrated nickel.
Flint: White or grey.
Holly Blue/Calapooia Purple: Purple stones found in Oregon.
Jasper: Opaque Red (iron oxide) or green (actinolite or chlorite), lacework of other minerals inside.
Onyx: Straight bands of black and white.
Sard: Brown
Thunderegg: A thunderegg is a geode (cavity inside another rock that is filled with crystals or chalcedony) that gets completely filled, usually with a type of chalcedony.

Where to it find in…

The Northwest: Calapooya River, OR (Holly Blue), other varieties can be found around most streams/beaches.
Other: Found all over the world in different varieties. Veins will be wildly different from location to location due to different mineral additives.
Past Uses: Has been used for jewelry and to carve. Arrowheads were often made from Chalcedony.
How it was Made: Forms in cavities and cracks, mainly, as silica rich water percolates through volcanic rocks. Also can come to replace other minerals as they dissolve (petrified wood).
Interesting Facts: You can strike a spark, not just from flint, but from any of the types of Chalcedony. Also, due to its’ slightly porous nature chalcedony can be dyed, and many of the commercial rocks for sale have been.