Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Chalcedony is a cryptocrystalline form of silica, consisting of very fine intergrowths of quartz and moganite (a monoclinic polymorph of silica). It is typically translucent to opaque and exhibits a waxy to dull luster. Its color is highly variable, ranging from white, gray, and blue to brown, red, and green, often due to the presence of various mineral inclusions or trace elements. It is characterized by its conchoidal fracture and lack of distinct cleavage.
How to Identify
- Color
- Highly variable; commonly white, gray, blue, brown, red, orange, green. Often banded or mottled.
- Luster
- Waxy to dull, sometimes vitreous on fresh fracture surfaces.
- Texture
- Smooth to slightly granular, often with a botryoidal (grape-like) or mammillary (breast-like) habit.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Often forms botryoidal, mammillary, stalactitic, or nodular masses, or as vein fillings and amygdules.
- Cleavage
- None. Exhibits a conchoidal fracture.
- Geological Environment
- Found in amygdaloidal cavities in volcanic rocks (e.g., basalt, rhyolite), as vein fillings, in geodes, as concretions in sedimentary rocks (e.g., limestone, shale), and as a replacement mineral in petrified wood.
Key Facts
- Hardness: 6.5 - 7 (Mohs scale)
- Specific Gravity: 2.55 - 2.65
- Crystal System: Trigonal (cryptocrystalline aggregates of quartz and moganite)
- Color: White, gray, blue, brown, red, orange, green; often banded or mottled
- Luster: Waxy to dull, sometimes vitreous
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Silicon dioxide (SiO₂)
Quick Check
- Color: Variable (white, gray, blue, brown, red, green)
- Luster: Waxy to dull
- Streak: White
Physical Characteristics
- Crystal Habit: Cryptocrystalline aggregates, typically botryoidal, mammillary, stalactitic, nodular, or as vein fillings. Individual crystals are microscopic.
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Waxy to dull, sometimes vitreous
Formation
Chalcedony forms from silica-rich solutions at relatively low temperatures and pressures. It typically precipitates in cavities, fissures, and amygdules within igneous rocks (especially volcanic rocks like basalt and rhyolite), and as a cementing agent or replacement material in sedimentary rocks. It can also form in hydrothermal veins and as a secondary mineral in weathered zones. The cryptocrystalline structure results from the rapid precipitation of silica, often as a gel, which then dehydrates and crystallizes into microscopic quartz fibers.
Usage
Chalcedony has been used since antiquity for tools, carvings, seals, and jewelry. Today, it is widely used as an ornamental stone, in lapidary arts, for cabochons, beads, and small sculptures. Specific varieties like agate and jasper are popular for decorative items. Due to its hardness and resistance to chemical attack, it is also used in some industrial applications, such as precision instrument bearings and mortar and pestles.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, often associated with volcanic activity and sedimentary processes.
Where to Find
Brazil
Known for large deposits of agate and other chalcedony varieties, particularly in Rio Grande do Sul.
Uruguay
Famous for its high-quality agate and amethyst geodes, often lined with chalcedony.
India
Historical source of carnelian and other chalcedony types.
United States
Various states, including Oregon (thunder eggs), Arizona (petrified wood, fire agate), California, and Washington, have significant chalcedony occurrences.
Madagascar
Produces a wide range of chalcedony, including colorful agates and jaspers.
Finding Tips
Look in Volcanic Terrains
Search for chalcedony in areas with ancient or recent volcanic activity, particularly in basalt flows where it fills vesicles (amygdules).
Check Sedimentary Rocks
Examine limestone, shale, and sandstone formations for chalcedony nodules, concretions, or petrified wood.
Explore Geodes and Veins
Chalcedony often lines or fills geodes and hydrothermal veins. Look for rounded or irregular rock formations that might contain internal cavities.
Observe Surface Luster
Chalcedony often has a distinctive waxy or dull luster, which can help distinguish it from other minerals in the field.
Test Hardness
Chalcedony is relatively hard (7 on Mohs scale), so it will scratch glass and steel. This helps differentiate it from softer minerals.
Similar Rocks
Quartz
Quartz (SiO₂)
Also known as: Rock Crystal, Amethyst, Citrine, Rose Quartz, Smoky Quartz
Opal
Opal (SiO₂·nH₂O)
Also known as: Precious Opal, Common Opal
Chert
Chert (SiO₂)
Also known as: Flint, Radiolarite
Scientific Classification
- Mineral Class
- Silicate (Tectosilicate)
- Group
- Quartz Group
- Crystal System
- Trigonal (as quartz component), Monoclinic (as moganite component)
- Chemical Formula
- SiO₂
- Composition
- Silicon dioxide
Explore Chalcedony
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.