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Prasiolite is a macrocrystalline variety of quartz, characterized by its pale green to yellowish-green color. Most prasiolite available on the market is produced by the heat treatment of amethyst or yellowish quartz. Natural prasiolite is considerably rarer. It shares all the fundamental physical and chemical properties of quartz, differing only in its color due to specific iron impurities and their alteration by heat or irradiation. The color can fade if exposed to strong sunlight or high temperatures for extended periods.
How to Identify
- Color
- Pale green to yellowish-green. The color is typically uniform but can sometimes show subtle zoning.
- Luster
- Vitreous (glassy).
- Texture
- Smooth to conchoidal fracture surfaces when broken. Crystalline forms are typically prismatic.
- Crystal Form
- Hexagonal prisms terminated by rhombohedra. Often found as massive or granular aggregates, or as individual crystals within geodes or veins.
- Cleavage
- None. Quartz lacks cleavage.
- Geological Environment
- Found in igneous, metamorphic, and sedimentary rocks. Specifically, natural prasiolite is found in areas where amethyst-bearing rocks have been subjected to natural heating events, such as volcanic or geothermal activity. Artificially produced prasiolite originates from amethyst deposits.
Key Facts
- Hardness: 7 on Mohs scale
- Specific Gravity: 2.65 g/cm
- Crystal System: Trigonal
- Color: Pale green to yellowish-green
- Luster: Vitreous
- Transparency: Transparent to translucent
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Pale green to yellowish-green
- Luster: Vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often terminated by rhombohedra; also massive, granular, cryptocrystalline (chalcedony form, though less common for prasiolite).
- Cleavage Type: None (absence of cleavage planes).
- Fracture Type: Conchoidal (shell-like, curved fractures).
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
Formation
Prasiolite is a green variety of quartz, typically formed by the heat treatment of amethyst or yellowish quartz containing iron-bearing mineral inclusions. Natural prasiolite is rare and forms when amethyst is heated by natural geological processes (e.g., geothermal activity, igneous intrusions) or irradiated. The green color is attributed to iron impurities (Fe2+ ions) within the quartz crystal lattice, which are altered by heat or irradiation. The specific temperature range for amethyst to turn green is generally between 450C and 500C. If heated above 500C, it can turn yellow (citrine) or colorless.
Usage
Primarily used as a gemstone in jewelry. Due to its attractive green color and quartz's durability, it is popular for rings, pendants, earrings, and other ornamental items. It is also collected by mineral enthusiasts.
Age Distribution
Geologically widespread, formation can occur in various geological ages.
Where to Find
Montezuma, Minas Gerais, Brazil
One of the primary sources of natural prasiolite. Amethyst from this region is known to turn green upon natural heating.
Lower Silesia, Poland
Another notable locality for natural prasiolite, where amethyst has been naturally altered.
Thunder Bay, Ontario, Canada
Some amethyst from this region can be heat-treated to produce prasiolite.
Various Amethyst Localities Worldwide
Since most prasiolite is heat-treated amethyst, any significant amethyst deposit (e.g., Uruguay, Bolivia, Zambia, Russia) can be a source for material that can be converted to prasiolite.
Finding Tips
Distinguish from other green minerals
Prasiolite's hardness (7 on Mohs scale) and lack of cleavage are key identifiers. Peridot is softer (6.5-7) and has distinct cleavage. Green beryl (emerald) is harder (7.5-8) and has imperfect cleavage. Glass imitations are softer and may show gas bubbles.
Observe color stability
Be aware that some prasiolite's color can fade with prolonged exposure to strong sunlight or heat. This is particularly true for artificially treated material. Natural prasiolite tends to be more color-stable.
Look for characteristic quartz crystal forms
When found in its natural crystalline form, look for the typical hexagonal prismatic habit with pyramidal terminations characteristic of quartz.
Check for inclusions
Like other quartz varieties, prasiolite can contain various inclusions, though high-quality gem material is typically eye-clean. Inclusions can sometimes help in origin determination.
Similar Rocks
Amethyst
Quartz (SiO2) variety Amethyst
Also known as: Purple Quartz
Citrine
Quartz (SiO2) variety Citrine
Also known as: Yellow Quartz
Peridot
Olivine ((Mg,Fe)2SiO4)
Also known as: Olivine
Green Beryl
Beryl (Be3Al2Si6O18)
Also known as: Emerald (if gem quality), Heliodor (if yellowish-green)
Scientific Classification
- Mineral Class
- Silicate mineral
- Group
- Tectosilicate (Quartz group)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide
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