Opal, Carnelian

Part 1: Carnelian (The Gemstone Profile)

Carnelian

A red variety of chalcedony.

Etymology:

While “Carnelian” is the prevailing modern term, it stems from the 16th-century corruption of “Cornelian” (along with related spellings such as Corneline or Cornalin). The term Cornelian, which shares cognates across several Romance languages, is derived from the Medieval Latin corneolus, originating from the Latin cornum (the Cornelian cherry), whose translucent red fruits resemble the gemstone. The Oxford English Dictionary notes that “Carnelian” is a corruption of “Cornelian,” influenced by a later analogy to the Latin caro or carnis (“flesh”).

Part 2: Opal (Scientific Data Sheet)

Physical Properties:

• Color: Colorless, white, yellow, red, orange, green, brown, black, blue.


• Luster: Vitreous, waxy, greasy, dull.


• Hardness (Mohs): $5.5 - 6.5$


• Specific Gravity: $1.9 - 2.3$

Etymology:

The origin of the name is uncertain. It may derive from the Sanskrit word “upala”, meaning “stone” or “precious stone,” or from “opalus”, the ancient Latin name for this gem (Pliny the Elder, 75–79 AD). While Pliny may have referred to this gem as “pedrus”, a modern interpretation by Kostov (2008) questions whether this term was indeed applied to what we recognize as opal in the modern sense.

Mineralogical Classification:

Although still considered a mineral species for historical reasons (2007), opal is not strictly defined as a true mineral because it varies from poorly crystalline cristobalite and/or tridymite to X-ray amorphous silica. Due to its amorphous nature, it is typically classified as a mineraloid, though its common but variable paracrystalline components make it more akin to a mixture.

Structural Classification:

Opal is structurally classified into three primary types, though they exist on a continuum (Jones & Segnit, 1971; Curtis et al., 2019):

1. Opal-CT: Originally thought to contain cristobalite and tridymite; it likely consists primarily of poorly crystalline tridymite with amorphous silica, occasionally accompanied by poorly crystalline cristobalite.


2. Opal-C: Composed of poorly crystalline sub-cristobalite material.


3. Opal-A: X-ray amorphous, which can be subdivided into:

• Opal-AG (Amorphous Gel): Consists of compacted amorphous silica spheres that occasionally form a diffraction lattice, creating the play-of-color seen in precious opal. The suffix “G” stands for “gel-like” due to its structural packaging (Flörke et al., 1991; Graetsch, 1994).


• Opal-AN (Amorphous Network): Usually found as hyalite. It still contains spheres, but they are significantly smaller (Li et al., 2022) and lack play-of-color.

Advanced Mineralogical Notes:

• Phase Transitions: Transitions between Opal-A, Opal-CT, and Opal-C are common (e.g., Curtis et al., 2019).


• Hydration Structure: Low-temperature studies suggest that water molecules may adopt an ice-like organization, including a cubic ice modification (Eckert et al., 2015).


• Nanostructure & Optical Properties: Li et al. (2022) demonstrated that Opal-AN consists of aggregates of varying nanosphere sizes (less than 5 nm), forming band characteristics. In contrast, studied Opal-AG contains compacted silica nanospheres with diameters of approximately 400 nm. These spheres can be arranged in ordered layers that scatter light, resulting in the characteristic play-of-color in most precious opals (Jones et al., 1964; Sanders, 1964, 1968).


• Precious Opal: Most precious opals are Opal-AG, although some are classified as Opal-CT (Curtis et al., 2019). The resulting diffraction structure and opacity may partially persist when Opal-AG undergoes crystallization into Opal-CT (Sanders, 1975).