Dark Colored Quartz sold as a Vivianite
Author: Deepa Srinivasa, Chief Gemologist, GSI
Introduction
Often a parcel of loose gemstones at a discounted price is tempting. Often, these parcels contain multiple gem categories, often mixed with different varieties of the same species or color.
Gemstone parcels are usually assorted according to size, color, clarity, and variety. Prices vary according to the assortment. Such deals often do not pose the opportunity to inspect every gemstone. Gemological laboratories also grab such deals and buy such parcels for research and study the type of gems being sold on the market. One such deal was a parcel with vivianite. These were deep blue cabochons with semi-translucent to opaque transparency weighing between 3.00 to 6.00 carats with each piece displaying fibrous appearance
Background
Abraham Gottlob Werner named vivianite in honor of John Henry Vivian (1785-1855), an English mineralogist and mine owner who first discovered the mineral in Cornwall (county in Southwest England). Fig 1
Fig 1: A natural Vivianite with a strong bluish green color. Image @ GSI
Vivianite is a hydrated iron phosphate mineral with a chemical composition of (Fe2+ Fe2+ 2(PO4)2·8H2O) and has been found in a number of geological environments. Minor amounts of manganese Mn2+, magnesium Mg and calcium Ca substitutes for iron Fe2+ in the structure. Vivianite is colorless when it’s pure but oxidizes easily, changing the color when iron becomes ferric.
Occurrence & Appearance
Vivianite occurs as crystals, masses or concretions in multiple geological environments such as organically rich sedimentary deposits or hydrothermal deposits. It may even be found inside fossil shells. From colorless in its pure state, vivianite can be very pale green to blue. Transparency ranges from transparent to translucent, and often show bladed or fibrous structure. Fig 2
Fig 2: (Left to right) An elongated rough of natural Vivianite with step-like fracture (1), base of the rough displayed mirror-like luster (2), the upper tip displayed eye-visible strong blue pleochroism (3 & 4). Image @ GSI
Upon exposure to light, vivianite oxidizes easily to darker green, brown, purple, or purplish black due to chemical transformation of the iron. This transformation of iron to the ferric state is usually responsible for causing deeper color or reducing transparency. Continuous exposure to light may turn the gem totally black and often totally opaque too.
Vivianite is fragile and soft, and this poses challenges during the lapidary process. Not very commonly used in jewelry, vivianite is usually found in collector’s mineral collections.
Material Tested
The parcel labeled as “vivianite”, purchased at one of the gems shows was used for a detailed study at GSI R&D Laboratory, Mumbai. The two samples tested weighed 3.00 and 2.60 carats, both cabochon cut, dark blue in color and semi-translucent transparency. The surface appeared to have chatoyancy or fibrous appearance when examined with a spotlight source. Fig 3. These samples were compared to the new samples of vivianite bought at the Tucson Gem Show in February 2023 (image 1). Basic gemological testing of both samples were done. Table 1
Fig 3: A dark blue Oval Cabochon with wavy fibrous structure sold as Vivianite. Image @ GSI
Fig 3: A dark blue Oval Cabochon with wavy fibrous structure sold as Vivianite. Image @ GSI
Magnification
The sample in question revealed fibrous appearance and fractures. The fibrous appearance with vague chatoyancy, RI and SG raised an alarm that the sample wasn’t actually vivianite. Fig.4
Fig 4: (left to right) A dark blue Oval Cabochon with wavy fibrous structure and wavy chatoyancy, not visible clearly. Image @ GSI
On the contrary natural vivianite samples in the parcel had numerous black needle like inclusions, step-like fracture and strong blue pleochroism. (Fig 5)
Fig 5: Magnification revealed black needle like inclusions (left), strong blue pleochroism can also be seen (right). Image @ GSI
Raman 532 nm
The suspected sample and natural vivianite were further tested on confirm the suspected materials identity. The sample was compared against the spectra from RRUFF and natural vivianite sample on hand.
Fig 6: Raman Spectra of Vivianite. Image @ RRUFF
Suspected Sample & Natural Vivianite Raman Spectra
Fig 7: Raman Spectra of Vivianite Imitation (Quartz) and Natural Vivianite. Image @ GSI
EDXRF
Both specimens (suspected and natural vivianite) chemical analysis were performed on EDXRF (Table 2). The graph indicated only Si in suspected sample (fig. 8) and natural vivianite indicated presence of Fe, Mn & P (fig. 9).
Suspected Sample: SiO2
Fig 8: EDXRF of Vivianite Imitation (Quartz) confirms only Si & Fe. Image @ GSI
Vivianite: (Fe2+ Fe2+ 2(PO4)2·8H2O)
Fig 9: EDXRF of Natural Vivianite confirms P, Fe & Mn. Image @ GSI
Vivianite: (Fe2+ Fe2+ 2(PO4)2·8H2O)
Table 2: EDXRF Results of Imitation & Natural Vivianite. Image @ GSI
Result & Conclusion
The data collected of the suspected sample confirmed the sample as quartz and the fibrous appearance along with color indicated hawk’s eye quartz. By contrast, hawk’s eye quartz is a macrocrystalline quartz colored by titanium. It is formed by quartz replacing the mineral crocidolite.
Whilst discounted prices are tempting, its important as a seller and buyer to disclose the true identity of the gemstones at every step of the supply chain. Always test each gemstone in your parcel or submit it to reputable gemological laboratory like GSI.