16.5.7: Sunset Quartz

Sunset Quartz
Chemical composition	SiO2 Silicon dioxide
Crystal system	Trigonal
Habit	Prismatic
Cleavage	Poor
Fracture	Conchoidal
Hardness	7
Optic nature	Uniaxial +
Refractive index	1.544 - 1.553
Birefringence	0.009
Dispersion	0.013
Specific gravity	2.65
Lustre	Vitreous
Pleochroism	None

Sunset quartz is a variety of quartz in very limited supply. It was discovered in 2006 in Minas Gerais, Brazil, with the vein producing only 5 kg. of gem quality material total. The collection pictured above, belonging to Karim Moussally, is one of the largest personal collections in existence.
The yellowish and orange color of the quartz is the result of microscopic acicular (needle-like) inclusions.

Figure \(\PageIndex{1}\): Acicular Incusions in Sunset Quartz: 40X
Photo courtesy of Barbra Voltaire

Analysis of these inclusions, reported in Gems & Gemology in Winter 2006, determined that they were NOT rutile as we might have suspected but were composed of Si, Al and Fe with traces of K! The identification of the actual mineral was inconclusive, but it has been speculated that it could possibly be acicular limonite.
The material is semi-transparent to translucent with a milky consistency.
The RI, Birefringence, SG (see template on the right) is typical of quartz.

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