Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Chalcedony is a cryptocrystalline variety of silica, meaning its crystals are too fine to be seen without high magnification. It is composed of very fine intergrowths of quartz and moganite. It typically exhibits a waxy to dull luster and can be translucent to opaque. Its color is highly variable, ranging from white, gray, and blue to red, orange, green, and brown, often due to the presence of various mineral inclusions or trace elements. Chalcedony is known for its toughness and resistance to weathering.
How to Identify
- Color
- Highly variable; white, gray, blue, brown, red, orange, green, black. Often uniform or banded.
- Luster
- Waxy to dull, sometimes vitreous on fresh fracture surfaces.
- Texture
- Smooth to slightly granular, often conchoidal fracture surfaces.
- Crystal Form
- Typically massive, botryoidal, mammillary, stalactitic, or as vein fillings. Individual crystals are microscopic.
- Cleavage
- None, due to its cryptocrystalline nature.
- Geological Environment
- Found in amygdules and veins in volcanic rocks (e.g., basalts, andesites), as nodules and concretions in sedimentary rocks (e.g., limestones, shales), and as a replacement mineral in petrified wood and fossils. Also occurs in hydrothermal veins.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 2.58-2.64
- Crystal System: Trigonal (for quartz component), Monoclinic (for moganite component)
- Color: Highly variable; white, gray, blue, brown, red, orange, green, black
- Luster: Waxy to dull, sometimes vitreous
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Silicon dioxide (SiO2), often with trace impurities
Quick Check
- Color: Variable (white, gray, blue, brown, red, green)
- Luster: Waxy to dull
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline, massive, botryoidal, mammillary, stalactitic, nodular, vein-filling
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle to tough
- Luster Type: Waxy to dull, sometimes vitreous
Formation
Chalcedony forms from the precipitation of silica-rich fluids at low temperatures and pressures. It typically occurs in cavities, veins, and as replacement material in various rock types. The silica precipitates as microscopic quartz fibers, often intergrown with moganite (a monoclinic polymorph of silica), forming a dense, microcrystalline aggregate.
Usage
Chalcedony has been used since antiquity for tools, carvings, and ornamental objects. Today, it is widely used in jewelry (cabochons, beads), decorative items, and as a lapidary material. Specific varieties like agate and jasper are popular for their aesthetic patterns and colors. Due to its hardness and resistance to chemical weathering, it is also used in some industrial applications, though less commonly than macrocrystalline quartz.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, often associated with volcanic activity or sedimentary processes.
Where to Find
Brazil
Significant source of various chalcedony varieties, including agate and amethyst geodes.
Uruguay
Renowned for high-quality agate and amethyst.
India
Historical source of carnelian and other chalcedony types.
Madagascar
Produces a wide range of chalcedony, including banded agate and jasper.
United States (Oregon, Washington, California, Arizona)
Various localities yield agate, jasper, and other chalcedony forms, often associated with volcanic terrains.
Germany
Idar-Oberstein region is historically famous for agate cutting and polishing.
Finding Tips
Look in Volcanic Terrains
Chalcedony often forms in gas cavities (amygdules) within basaltic lava flows. Look for rounded or irregular nodules.
Check Sedimentary Rocks
Examine limestone and shale outcrops for chert nodules or concretions, which are forms of chalcedony.
Explore Riverbeds and Gravel Pits
Due to its hardness and resistance to weathering, chalcedony often survives transport and can be found as pebbles or cobbles in alluvial deposits.
Identify Waxy Luster
A characteristic waxy to dull luster on a fresh fracture surface can be a good indicator.
Test Hardness
Chalcedony has a Mohs hardness of 6.5-7, meaning it will scratch glass and steel.
Similar Rocks
Quartz
SiO2
Also known as: Rock Crystal, Amethyst, Citrine, Rose Quartz, Smoky Quartz
Opal
SiO2·nH2O
Also known as: Precious Opal, Common Opal
Chert
Cryptocrystalline Quartz
Also known as: Flint
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group
- Crystal System
- Trigonal (quartz) and Monoclinic (moganite)
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide, often with minor amounts of water and various impurities (e.g., iron oxides, nickel, chromium, copper) that impart color.
Explore Chalcedony
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.