What is peruvian opal?
Peruvian opal is a mineraloid hydrated silica (SiO2·nH2O) formed in the Andes Mountains of Peru, distinguished by its semi-opaque to translucent blue-green coloration and unique dendrite inclusions. This volcanic-originated gemstone exhibits a hardness of 5.5-6.5 on the Mohs scale and possesses a vitreous to resinous luster.
The stone forms in volcanic host rocks through low-temperature hydrothermal processes, where silica-rich solutions fill cavities and fractures. Unlike precious opal, Peruvian opal lacks play-of-color but contains trace elements of copper and palygorskite clay minerals, contributing to its characteristic blue-green hue. The material often displays dark, branching manganese oxide dendrites, creating distinctive landscape-like patterns within the stone’s structure.
Mining occurs primarily in the Cajamarca region of northern Peru, where deposits formed during the Miocene epoch approximately 23-5.3 million years ago. The stone’s composition includes 3-9% water content, contributing to its unique optical and physical properties.
Type of opal (natural vs synthetic)
Peruvian opal is typically a natural gemstone.
Synonyms / common names
Common names for Peruvian opal include Andean opal, Peruvian blue opal, and Peruvian pink opal.
Hardness:
Peruvian opal has a hardness rating of approximately 5.5 to 6.5 on the Mohs scale, which makes it relatively soft compared to more commonly known gemstones like diamonds or sapphires.
Refractive index (ri):
The refractive index of Peruvian opal typically ranges from 1.42 to 1.45, which is relatively low and contributes to its subtle visual effects compared to other more brilliant gemstones.
Luster:
This gemstone exhibits a waxy to vitreous luster, giving it a soft and appealing glow that is quite distinct from the sparkle of harder stones.
Cleavage:
Peruvian opal does not have defined cleavage, which makes it somewhat more durable against breaking along cleavage planes when being cut or polished.
Fracture:
It usually has an uneven to conchoidal fracture, contributing to its unique and natural appearance when cut and polished.
Specific gravity (sg):
The specific gravity of Peruvian opal ranges from about 2.10 to 2.20, which is typical for opals and indicates a relatively lower density compared to many other gemstones.
Optical birefringence (double refraction):
Peruvian opal is an amorphous gemstone, lacking a crystalline structure, thus it does not exhibit double refraction.
Dispersion (fire):
The dispersion or fire of Peruvian opal is generally low. It does not display the significant play of color seen in precious opals but may show a subtle color play in high-quality specimens.
Crystal system:
As an opal, Peruvian opal is amorphous and does not have a crystal system.
Color:
Peruvian opal comes in a range of colors, most commonly in shades of blue and green. Less commonly, it can be found in pink. Its colors are typically soft and pastel.
Transparency:
This type of opal ranges from translucent to opaque, which impacts its visual depth and appeal.
Pleochroism:
Pleochroism is not observed in Peruvian opal due to its amorphous nature.
Fluorescence:
Peruvian opal generally does not show fluorescence under UV light.
Toughness:
Peruvian opal has fair to good toughness, making it reasonably suitable for jewelry that does not receive rough wear.
Tenacity:
The tenacity of Peruvian opal is generally brittle, which is a common characteristic of opal varieties.
Optic sign:
Being amorphous, Peruvian opal is isotropic and shows no optic sign.
Absorption spectrum:
Peruvian opal does not typically display a distinctive absorption spectrum.
Chemical composition:
Peruvian opal is a form of hydrated silica (SiO2·nH2O). Its water content may range from 3% to 20%.
Chatoyancy (cat‚äôs eye effect):
While rare, some forms of Peruvian opal can exhibit chatoyancy, especially if cut en cabochon and when fibrous inclusions are present.
Asterism (star effect):
Asterism or the star effect is generally not observed in Peruvian opal.
Iridescence:
Iridescence is typically not a feature of Peruvian opal, unlike the play of color seen in precious opals.
Magnetism:
Peruvian opal is not magnetic.
Electrical conductivity:
Being a type of silica, Peruvian opal has poor electrical conductivity.
Radioactivity:
Peruvian opal is not radioactive.