Black Agate
Black Chalcedony
A variety of Chalcedony
What is Black Agate?
Black Agate is a type of microcrystalline quartz that is opaque and appears black or dark gray in color. However, the gemstone of black Agate in the market is usually created by suffusing chalcedony in a sugar solution and then treating with sulfuric acid, causing the stone's appearance to change to a black color. This process enhances the stone's appearance, making it very popular. It is commonly used as a Jet imitation.
Optical Properties
- Refractive Index
- 1.530-1.543
- Birefringence
- 0.003-0.009
- Pleochroism
- None
- Optical Character
- Biaxial positive
- Dispersion
- 0.013
Discover Values
Market Value Factors
Pricing varies for every rock and mineral, so use these universal factors to gauge Black Agate before comparing listings or appraisals.
Size & Weight
Larger, intact specimens usually command higher prices.
Rarity & Demand
Scarce material or popular varieties sell at a premium.
Condition & Finish
Chips, repairs, and heavy wear lower value; clean prep helps.
Treatment & Provenance
Untreated specimens with documented locality are prized.
Where to Find Black Agate
Black Agate is a variety of Chalcedony. RockHoundR, the free-to-download rockhounding map from the Ruby Glint team, shows where collectors find chalcedony in the US, with land-access notes for BLM land and national forests. Collecting is banned in national parks.
See where to find chalcedonyAlways check the local rules and get the landowner's permission before you collect.
Health & Safety Information
- ⚠️Black Agate in powdered form can be toxic due to its Crystalline Silica content.
- ⚠️Quartz, silica, crystalline silica and flint are non-toxic materials, but very fine dust containing quartz, known as respirable crystalline silicon (RCS), can cause serious and fatal lung disease. Lapidaries should exercise caution when cutting silica.
- ⚠️Black Agate is typically non-toxic unless it's in powder form. Long-term exposure to finely ground powder may lead to silicosis. Therefore, when cutting or polishing black Agate, be sure to wear a dust mask to avoid inhaling its dust.
Quick Facts
Physical Properties
- Color
- Black
- Hardness (Mohs)
- 6.5 - 7
- Rock tumbling
- Tumbles well
- Density
- 2.55 - 2.7 g/cm³
- Streak
- White
- Luster
- Vitreous, Waxy
- Crystal System
- Trigonal
Chemical Properties
- Chemical Formula
- SiO2
- Elements
- O, Si
Also Known As
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Black Agate FAQs
How do I identify Black Agate?
Black Agate can be identified by its hardness of 6.5 - 7 on the Mohs scale, Black color, Vitreous, Waxy luster, Trigonal crystal system. Look for these key characteristics when examining specimens.
What color is Black Agate?
Black Agate typically appears in Black. Color can vary depending on impurities and formation conditions.
How hard is Black Agate?
Black Agate has a hardness of 6.5 - 7 on the Mohs scale. This gives it moderate hardness.