Pitch Opal

What is pitch opal?

Pitch opal is a rare variety of common opal characterized by its opaque black body color and complete absence of play-of-color, formed through the natural inclusion of carbonaceous materials within its amorphous silica structure. This distinctive gemstone occurs primarily in Lightning Ridge, New South Wales, Australia, where it forms in ancient sedimentary deposits.

The stone exhibits a unique combination of physical properties, including a refractive index of 1.44-1.46, a specific gravity between 1.98-2.20, and a hardness of 5.5-6.5 on the Mohs scale. The carbonaceous content responsible for its pitch-black appearance consists primarily of disseminated organic matter and microscopic carbon particles trapped during opal formation. The gem’s internal structure comprises densely packed spheres of silica, which, unlike precious opal, are arranged in a pattern that prevents the diffraction of light necessary for iridescence.

Type of opal (natural vs synthetic)

Pitch opal is typically a natural gemstone.

Synonyms / common names

Common names for Pitch opal include Fire opal, Girasol opal, and Hyalite.

Hardness:

Pitch opal, like other opal varieties, has a relatively low hardness, typically ranging between 5.5 to 6.5 on the Mohs scale. This makes it relatively softer compared to many other gemstones.

Refractive index (ri):

The refractive index of pitch opal generally falls between 1.44 and 1.46, which is characteristic of opals. This property contributes to its unique play of color.

Luster:

Pitch opal exhibits a vitreous to resinous luster, which can enhance its visual appeal and the vibrancy of its colors.

Cleavage:

Opals, including pitch opal, do not have a defined cleavage. This means they do not break along well-defined, flat planes.

Fracture:

The typical fracture in pitch opal is conchoidal, which is a smoothly curving (shell-like) breakage pattern. This is common among amorphous materials like opal.

Specific gravity (sg):

The specific gravity of pitch opal typically ranges around 2.15, which is in the standard range for opals. This measure indicates the density of the gemstone compared to water.

Optical birefringence (double refraction):

Pitch opal, being an amorphous mineraloid, does not exhibit double refraction. It is isotropic, meaning it has the same optical properties in all directions.

Dispersion (fire):

Opals are well known for their spectacular dispersion or “”fire,”” and pitch opal is no exception. This property refers to the stone‚Äôs ability to disperse light into multiple colors.

Crystal system:

Pitch opal does not have a crystal system as it is amorphous. This means it lacks a long-range, ordered atomic structure, which is typical of crystalline materials.

Color:

The color of pitch opal varies, but it often displays a dark body tone which can enhance the visibility of its play-of-color.

Transparency:

Pitch opal ranges from opaque to translucent, depending on the specific conditions under which it formed.

Pleochroism:

Pleochroism is not observed in pitch opal due to its amorphous nature and isotropic properties.

Fluorescence:

Pitch opal may exhibit fluorescence under ultraviolet light, typically showing a greenish or whitish glow, although this can vary.

Toughness:

The toughness of pitch opal is generally fair, but it is sensitive to cracking and chipping due to its relatively high water content and low hardness.

Tenacity:

Pitch opal is brittle, which is common for opals due to their internal structure and water content.

Optic sign:

Being isotropic, pitch opal does not have an optic sign.

Absorption spectrum:

Pitch opal does not typically display a characteristic absorption spectrum due to its amorphous nature and varied composition.

Chemical composition:

Pitch opal is primarily composed of silica (SiO2) and water, with the water content varying up to about 10% by weight.

Chatoyancy (cat‚äôs eye effect):

While not commonly associated with pitch opal, some specimens may exhibit a cat’s eye effect if they have aligned fibrous inclusions.

Asterism (star effect):

Asterism, or the star effect, is generally not observed in pitch opal due to its lack of crystalline structure.

Iridescence:

Pitch opal can display a striking iridescence, particularly evident in the form of opalescence or the play of color for which opals are famous.

Magnetism:

Pitch opal is not magnetic, aligning with the typical properties of silicate minerals which lack ferromagnetic materials.

Electrical conductivity:

Pitch opal is generally a poor conductor of electricity, consistent with its composition as a silicate mineraloid.

Radioactivity:

Pitch opal is not radioactive and poses no health risks related to radioactivity.