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Prasiolite is a silicate mineral, a green variety of quartz (SiO2). It is characterized by its pale to medium green color, which can range from yellowish-green to a more vibrant leek-green. Most prasiolite available commercially is produced by the heat treatment of amethyst or yellowish quartz, primarily from Brazil and Poland. Natural occurrences are rare. It exhibits the typical physical and chemical properties of quartz, including a hexagonal crystal system, vitreous luster, and a Mohs hardness of 7. Its color is stable under normal conditions but can fade if exposed to high temperatures or intense sunlight for prolonged periods.
How to Identify
- Color
- Pale to medium green, often with a yellowish tint. Can be distinguished from other green gemstones by its specific shade and lack of strong pleochroism.
- Luster
- Vitreous (glassy).
- Texture
- Smooth to conchoidal fracture surfaces when broken. Crystals are typically prismatic.
- Crystal Form
- Hexagonal prisms terminated by rhombohedra. Often found as massive or granular aggregates, or as well-formed crystals in geodes and veins.
- Cleavage
- None (lacks cleavage).
- Geological Environment
- Naturally occurring prasiolite is found in igneous and metamorphic rocks where amethyst has been subjected to natural heating. Artificially produced prasiolite originates from amethyst or yellowish quartz found in pegmatites, hydrothermal veins, and volcanic rocks.
Key Facts
- Hardness: 7 (Mohs scale)
- Specific Gravity: 2.65 g/cm³
- Crystal System: Trigonal (often described as hexagonal due to external morphology)
- Color: Pale to medium green, yellowish-green
- Luster: Vitreous
- Transparency: Transparent to translucent
- Fracture: Conchoidal
- Cleavage: None
- Composition: Silicon dioxide (SiO2)
Quick Check
- Color: Pale to medium green
- Luster: Vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Prismatic, often terminated by rhombohedra; also massive, granular, cryptocrystalline (chalcedony form is not typically prasiolite).
- Cleavage Type: None (lacks cleavage).
- Fracture Type: Conchoidal (shell-like, curved fractures).
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy).
Formation
Prasiolite is a green variety of macrocrystalline quartz. Its green color is typically induced by heat treatment of amethyst or yellowish quartz containing iron impurities. Natural prasiolite is rare and forms when amethyst-bearing rocks are heated by natural geological processes, such as metamorphism or igneous intrusions. The iron impurities within the quartz structure, often in the form of Fe3+ ions, are reduced to Fe2+ ions upon heating, leading to the green coloration. Irradiation can also play a role in color development or modification. The specific temperature range for artificial heat treatment is generally between 400°C and 500°C; exceeding this can cause the color to fade or turn brown.
Usage
Primarily used as a gemstone in jewelry. Due to its attractive green color and good durability, it is often faceted for rings, pendants, and earrings. It is also collected by mineral enthusiasts.
Age Distribution
Prasiolite itself is not dated by geological age, but rather the host rocks in which it forms. Quartz is ubiquitous in rocks of all ages, from Precambrian to Cenozoic.
Where to Find
Monteiro, Rio Grande do Sul, Brazil
A primary source of amethyst that is heat-treated to produce most commercial prasiolite.
Lower Silesia, Poland
Another significant source of amethyst and yellowish quartz used for heat treatment.
Thunder Bay, Ontario, Canada
Known for natural prasiolite occurrences, though these are rare and typically less intensely colored than heat-treated material.
Serra de CarnaíBA, Bahia, Brazil
Some natural prasiolite has been reported from this region.
Finding Tips
Look for Quartz Deposits
Prasiolite is a variety of quartz, so search for geological environments known to host quartz, such as pegmatites, hydrothermal veins, and volcanic geodes. Natural prasiolite is often found alongside amethyst.
Examine Color Carefully
Natural prasiolite tends to have a more subtle, often grayish-green or leek-green color, compared to the brighter, more uniform green of heat-treated material. Be aware that most commercial prasiolite is treated.
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 identifying the origin or natural vs. treated status.
Consider Hardness
As quartz, prasiolite will scratch glass and steel. This can be a quick field test to confirm it is a quartz variety, though it won't distinguish it from other quartz types.
Similar Rocks
Amethyst
Quartz (α-SiO2, purple variety)
Also known as: Purple Quartz
Citrine
Quartz (α-SiO2, yellow variety)
Also known as: Yellow Quartz
Peridot
Olivine ((Mg,Fe)2SiO4)
Also known as: Olivine (gem-quality)
Green Tourmaline
Elbaite (Na(Li1.5Al1.5)Al6(Si6O18)(BO3)3(OH)4)
Also known as: Verdelite
Scientific Classification
- Mineral Class
- Silicates
- Group
- Tectosilicates (Quartz Group)
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide, with trace iron impurities responsible for color.
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