Chalcedony Identification Guide
How to identify chalcedony, the microcrystalline quartz family, by its waxy luster, translucency, hardness 7, and how it relates to agate and jasper.
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What Chalcedony Looks Like
Chalcedony is the broad family name for microcrystalline (cryptocrystalline) quartz, made of fibrous, ultra-fine quartz crystals too small to see. In its narrow sense, "chalcedony" refers to the translucent, evenly colored varieties, typically white, gray, blue, or bluish, with a smooth, waxy to slightly glassy luster. It forms botryoidal (grape-like) crusts, nodules, vein fillings, and geode linings. Many familiar stones are chalcedony varieties: agate (banded), carnelian (orange-red), chrysoprase (green), onyx, and jasper (opaque).
Key Visual Traits
- Translucent, even color (commonly white, gray, blue)
- Waxy to dull-glassy luster
- Botryoidal, nodular, or massive habit (no visible crystals)
- Smooth conchoidal fracture
Step-by-Step Field-ID Checklist
- Check translucency. True chalcedony glows softly when a thin edge is held to light.
- Note the luster. Waxy to greasy, not metallic or earthy.
- Test hardness. Mohs 7; it scratches glass and resists a steel knife.
- Inspect the surface form. Look for botryoidal crusts or smooth nodular shapes with no visible crystal faces.
- Examine fracture. Conchoidal, smooth, sharp edges, no cleavage.
- Confirm inertness. No reaction to dilute acid.
Diagnostic Tests
- Hardness: 7 (scratches glass; knife will not scratch it).
- Fracture: Conchoidal, no cleavage.
- Streak: White.
- Density: About 2.58 to 2.64.
- Acid: Inert to dilute HCl.
- Transparency: Translucent (massive varieties) to opaque (jasper).
Common Look-Alikes and How to Tell Them Apart
- Opal: Softer (5.5 to 6.5, can be scratched by quartz), lower density, often shows play-of-color or a glassy hydrated look; chalcedony is harder.
- Calcite/onyx marble: Much softer (3), fizzes in acid; chalcedony is hard and inert.
- Glass: Often shows bubbles and a more brilliant luster; chalcedony has a waxy luster and natural banding or botryoidal form. Glass can be scratched by quartz.
- Jasper vs chalcedony: Jasper is the opaque, impurity-rich member of the same family; translucency separates them.
- Agate vs chalcedony: Agate is the banded variety; plain even-colored translucent material is called chalcedony.
Where It Is Typically Found
Chalcedony forms at low temperatures from silica-rich solutions in cavities and fractures of volcanic and sedimentary rocks, as well as in weathering environments. It is abundant worldwide; major sources include Brazil, Uruguay, India, Madagascar, Indonesia, Turkey, Namibia, and the western United States. It is collected as nodules, geode linings, seam fillings, and botryoidal crusts.
Collector and Field Notes
Chalcedony is one of the most widely dyed and treated lapidary materials, so inspect cut faces for color that pools unnaturally in cracks, a sign of dyeing. Natural blue chalcedony (such as Mojave or Namibian "blue chalcedony") is gently colored and evenly translucent. The material is extremely durable at hardness 7 and takes a fine polish, making it ideal for cabochons, beads, and carvings. Because so many trade names (agate, carnelian, chrysoprase, sard, onyx) all describe chalcedony varieties, identification often comes down to recognizing the family first, by waxy luster, translucency, and hardness, then naming the variety by color and banding.
Frequently asked questions
What is chalcedony?
Chalcedony is microcrystalline quartz, a family of cryptocrystalline silica that includes agate, carnelian, chrysoprase, onyx, and jasper. In its narrow sense it means the translucent, evenly colored (often white, gray, or blue) members.
How can you tell if it's real chalcedony?
Check for hardness 7 (scratches glass, resists a knife), a waxy translucent appearance, conchoidal fracture, no cleavage, and no reaction to acid. Botryoidal or nodular form with no visible crystals supports the identification.
Chalcedony vs agate: what's the difference?
Agate is simply the banded variety of chalcedony. Evenly colored, unbanded translucent material is called chalcedony, while concentric or parallel banding makes it agate. Both are the same microcrystalline quartz.
Chalcedony vs opal?
Chalcedony is crystalline quartz at hardness 7, while opal is hydrated amorphous silica at 5.5 to 6.5 that can be scratched by quartz. Opal is lighter and may show play-of-color, which chalcedony never does.
Chalcedony identified by the community
Recent Chalcedony specimens identified with Rock Identifier.
