16.5.1: Amethyst
- Page ID
- 4086
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|---|---|
| Chemical composition | Silicon dioxide SiO2 |
| Crystal system | Trigonal |
| Habit | Prismatic |
| Cleavage | Poor |
| Fracture | Conchoidal |
| Hardness | 7 |
| Optic nature | Uniaxial + |
| Refractive index | 1.544 - 1.553 |
| Birefringence | 0.009 |
| Dispersion | Low, 0.013 |
| Specific gravity | 2.63 - 2.65 |
| Lustre | Vitreous |
| Pleochroism | Weak to strong |
| Amethyst image gallery |
Amethyst is a purple variety of the mineral quartz. It occurs in all intensities of the color purple from a light pastel to a depth of royal splendor. Until the beginning of the 20th century, it was quite rare and costly. When vast deposits were found in Brazil, amethyst became very accessible and affordable. Amethyst has always been linked to the thinking process, ensuring clarity of vision. It inspires creativity, courage, and valor. Amethyst has been successfully synthesized in the lab, so buyers need to be sure their source is qualified to separate natural from lab-grown material.
Magnification
Amethyst is a type I stone in the GIA clarity grading system is usually free from eye visible inclusions. Typical inclusions visible with magnification are:
- liquid feathers "fingerprints" (also known as "tiger stripes" or zebra stripes")
- 2-phase, 3-phase inclusions
- negative crystals
Figure \(\PageIndex{1}\): classic "fingerprint" inclusion
30X Magnification
By Barbra Voltaire
Polariscope
With the use of a polariscope, one can find the typical "bull's eye" for quartz, but that can be seen in both twinned natural stones as in many twinned synthetics.
To discern the hydrothermal (twinned) synthetics from natural amethyst one must look at the interference figures of the stone without a conoscope. In natural, twinned, amethyst one will see a pattern known as "Brewster fringes". This can be seen when looking down the optic axis of the gem and on lateral rotation, this pattern quickly disappears. With the twinned hydrothermal stones these fringes are not seen in the typical triangular form, rather they follow the outline of the stone. On lateral rotation, this image will be in view much longer. This method works in most cases, for the remaining 1% to 3% one will need to infrared spectrometry to discern between the two.
Immersion in a liquid and magnification might aid greatly in spotting the "Brewster fringes".
G&G Articles on Amethyst 1934-1980
The GIA has published all the G&G's from 1934 until 1980 online. The organization of the list by subject was done by Joseph Gill.
- Winter 1949, Amethyst color induced in rock crystal via cyclotron, p. 255, 1p.
- Winter 1963, A very rare star amethyst, p. 101, 2pp.
- Summer 1971, Inclusions in amethyst that look like space capsules, p. 322, 2pp.
- Winter 1977, Synthetic Amethyst from Russia, p. 365, 2pp
- Winter 1978, Synthetic Amethyst, p. 365, 2pp.
- Spring 1979, Synthetic Amethyst, currently no test for a flawless amethyst, p. 151, 3pp.
- Summer 1980, Citrine-Amethyst Quartz - A Gemologically New Material, by John I. Koivula, p. 290, 4pp.
- Winter 1980, More News On Citrine-Amethyst Quartz, by John I. Koivula, p. 409, 1p.
References
- Notari F., Boillat P.-Y., Grobon C. (2001), Quartz alpha-SiO2: Discrimination des améthystes et des citrines naturelles et synthétiques, Revue de Gemmologie AFG, N° 141/142, pp. 75-80.