Shape	cushion
Size	19.0 x 15.5 x 10.5 mm
Color	reddish-orange/brown, almost dark red in incandescent light
Lustre	vitreous
Weight	20.14 ct
SG	2.65 [hessonite garnet: 3.57-3.67, quartz: 2.62-2.67]
RI	1.542 - 1.552 [hessonite garnet: 1,727-1,771, quartz: 1.543-1.554]
DR	0.008 - Uniaxe positive [hessonite garnet: isotropic, quartz: U+]
Pleochroism	none
Polariscope / Conoscope	light/dark 4 times / 360°, bull's eye interference pattern observed with conoscope (usually in quartz) - optical axis is || to the stone's length
SWUV	inert
LWUV	inert
Magnetic susceptibility N52	N52: (-0.06 ct) -> very slightly attracted [quartz is usually diamagnetic]
Chelsea filter	reddish to red

Table 1. Observational and measured properties

Optical and physical properties (table 1) do not match the hessonite garnet ones but they do exactly match that of quartz except for the magnetic susceptibility. Some citrine colored by iron can be weakly attracted by a magnet.
Optical anisotropy of material can be observed by looking at the doubling of inclusions and facet edges (figure 2). The stone, even if it looks rather clean, it is heavily included: highly reflective healed fracture (golden inclusions in figure 1), elongated solid or tube inclusion (figure 2), etc. While immersed in water (figure 3), the stone shows some highly reflective inclusions that are viewed black in transmitted light. The bottom-right quarter of the picture shows straight color-zoning in two directions and the bottom-left quarter a curved color-zoning for which the difference in color is noticeable. The later is difficult to detect without immersion.

Infrared reflectance spectroscopy:

The IR reflectance spectrum (figure 4), acquired from the table, exhibits the characteristic spectrum of the quartz  specie which is unambiguously different of that of hessonite grossular garnet.

UV-VIS-NIR spectroscopy:

The UV-Vis-NIR absorption spectrum (figure 5) was acquired with the light path perpendicular to the table and entering the stone at the culet point. The light path is therefore perpendicular to the C-axis (C-axis || stone's length). It shows a strong absorption below 600 nm while the light from the red to NIR region is almost unabsorbed. Such absorption pattern produces the dark-orange / reddish-brown color.

Citrine's color (yellow to orange-brown quartz variety) is possibly explained by the following causes:

• O^2-→Fe³⁺ charge transfer (Fritsch & Rossman 1988), microscopic iron particles around 100 nm in diameter as hematite (Fe₂O₃) ex-solution. The Uv-Vis-NIR absorption spectrum is characterized by an absorption edge as the one in figure 5 but with the main difference that it is located between 400 and 500 nm. Such a citrine are usually the result of the heat-treatment at 400-500° C of amethyst (the violet color variety of quartz) and does not show any pleochroism.
• Al-bearing quartz with Al³⁺ replacing Si⁴⁺ (silicon) where the charge difference is compensated by alkali ions such as Na, K, Li or H₃O⁺. The UV-Vis-NIR sabsorption spectrum is characterized by three rather flat bands in the visible domain centered at 670, 486 and 427 nm. These 3 bands are also accompanied by two bands in the UV domain at 314 and 270 nm. Such a citrine show a weak pleochroism of two different yellow color.

The figure 3 spectrum of this reddish-brown quartz is similar to that of citrine colored by iron except the absorption edge which is shifted to 500-600 nm. This is consistent with the unusual reddish-brown color of the stone.

Photoluminescence spectroscopy:

No photoluminescence observed with 254, 365, 405 nm sources.

Conclusion:

This 20.14 ct reddish-brown stone is a quartz that can be put in the citrine category because of its color. Quartz specie is indicated by the optical and physical properties of the classical gemology and confirmed by the IR spectroscopy. The strong reddish-brown color is likely caused by iron but it is not possible to state the color was obtained or not by heat-treatment of an amethyst.