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Welo opal is a precious hydrated silica gemstone exclusively mined from the Welo province in northern Ethiopia, characterized by its distinctive play-of-color and transparent to translucent body tone. The stone formed during the Oligocene to Miocene epochs within volcanic host rocks through the deposition of silica-rich solutions in cavities and fissures.
These Ethiopian opals exhibit exceptional clarity and display vibrant spectral colors against their predominantly white, yellow, or orange base. Their internal structure consists of uniformly-sized silica spheres arranged in a regular pattern, creating diffraction gratings that produce their characteristic opalescence. Welo opals possess a unique characteristic of being hydrophane, allowing them to temporarily absorb water and become more transparent.
The deposit, discovered in 2008, produces opals with stability superior to Australian varieties, showing minimal crazing or cracking over time. These gems typically contain 6-10% water content and register 5.5-6.5 on the Mohs hardness scale. Their specific gravity ranges from 1.98-2.20, with a refractive index between 1.43-1.46.
Welo opal is typically a natural gemstone.
Common names for Welo opal include Ethiopian opal, Wollo opal, and Queen of Gemstones.
Welo opal, like other opals, has a relatively low hardness rating, typically ranging between 5.5 and 6.5 on the Mohs scale. This makes it somewhat susceptible to scratches and abrasions if not handled with care.
The refractive index of Welo opal varies from about 1.37 to 1.52, which contributes to its famous play-of-color.
Welo opal exhibits a vitreous to waxy luster, enhancing its visual appeal and its vibrant color display.
Opals, including Welo opal, generally do not have cleavage. They are amorphous, meaning they lack a defined crystalline structure, which impacts how they break when cut or shaped.
The fracture of Welo opal is typically conchoidal, which means it breaks with a smooth, curved surface that resembles the interior of a seashell.
Welo opal has a specific gravity typically between 1.98 and 2.25, which is relatively lower compared to many other gemstones. This property affects the heft and feel of the gemstone.
As an amorphous gemstone, Welo opal does not exhibit double refraction. It is isotropic, showing the same optical properties in all directions.
Opal is known for its spectacular play-of-color, which is not exactly the same as dispersion but often referred to as “”fire.”” This optical phenomenon is caused by the diffraction of light passing through tiny silica spheres within the structure of the opal.
Opals, including Welo opal, are amorphous minerals. They do not belong to any crystal system as they lack a long-range crystalline structure.
The body color of Welo opal can range widely, from clear through white, gray, red, orange, yellow, green, blue, magenta, rose pink, slate, olive, brown, and black. The play-of-color can include a full spectrum of colors.
Welo opals can range from completely opaque to translucent or even transparent, depending on the conditions under which the opal formed.
Pleochroism is not observed in Welo opal due to its isotropic nature as an amorphous substance.
Welo opals can show strong fluorescence under ultraviolet light, typically displaying a green or white glow, which can be quite striking.
The toughness of Welo opal is generally fair to poor due to its relatively high water content and lack of crystalline structure, which makes it prone to drying and cracking.
Welo opal is generally considered brittle, which aligns with its fair to poor toughness. It can be sensitive to pressure and might crack or chip if handled roughly.
Being isotropic, the optic sign of Welo opal is not applicable as it shows the same optical properties in all directions and does not double refract light.
Welo opal does not typically show a characteristic absorption spectrum due to its amorphous nature and varied composition.
Welo opal is primarily composed of silica (SiO2) and water, with its water content sometimes reaching up to 10% by weight.
While relatively rare, some Welo opals can exhibit chatoyancy, also known as the cat’s eye effect, when cut properly and when fibrous inclusions are present.
Asterism, or the star effect, is not commonly observed in Welo opal due to its inherent structural properties.
One of the most defining features of Welo opal is its iridescence. The stone can display a range of spectral colors that shimmer and change with the angle of light reflection.
Welo opal is not magnetic, which aligns with its silica composition lacking ferromagnetic materials.
Welo opal is generally a poor conductor of electricity, primarily due to its high silica and water content.
Welo opal is not radioactive and poses no risks in terms of radioactivity to its wearers or handlers.
