Peridot is a gem-quality variety of the mineral olivine, a magnesium iron silicate (\(\text{(Mg,Fe)}_2\text{SiO}_4\)). This gemstone is distinctive for its single color, ranging from yellowish-green to deep olive, an attribute derived from the iron within its atomic structure. Because of its formation deep within the Earth’s mantle, peridot is a subject of interest in both gemology and geology. To accurately identify peridot, geologists rely on a variety of physical and optical tests, including the fundamental streak test.
Understanding the Streak Test
The streak test is a simple but powerful technique used to determine the color of a mineral’s powder, which is known as its streak. This color is often more consistent and diagnostic than the mineral’s external body color, which can be altered by trace impurities, weathering, or surface conditions. The streak color represents the mineral’s true color in its finely powdered form, revealing the inherent composition without the interference of crystal structure effects.
The test is performed by firmly dragging a specimen across an unglazed porcelain plate, commonly called a streak plate. This action grinds a small amount of the mineral into a fine powder, leaving a colored line on the rough ceramic surface. Minerals with a Mohs hardness greater than about seven typically scratch the porcelain plate rather than leaving a streak, making the test non-diagnostic for those very hard substances. For softer minerals, however, the resulting powder color provides reliable identification data.
The Specific Streak of Peridot
When peridot is subjected to the streak test, it yields a colorless or white streak. This result is common among silicate minerals and is directly tied to peridot’s relatively high hardness (6.5 to 7 on the Mohs scale) and its specific chemical makeup. This hardness means it is near the threshold where a mineral begins to scratch the porcelain plate rather than powdering easily.
The white streak occurs because the mineral’s primary chromophore, iron, is integrated within the silicate crystal structure. The iron is responsible for the macroscopic green color, but when the mineral is crushed into a fine powder, the resulting particles are too small to absorb and reflect light in a way that produces a strong color. Unlike minerals containing iron that create dark streaks, peridot’s composition does not translate to a distinct powder color.
The chemical formula \(\text{(Mg,Fe)}_2\text{SiO}_4\) shows peridot is composed of magnesium, iron, silicon, and oxygen. The silicate tetrahedra, which form the structural backbone, are colorless, and the iron ions do not create a strongly colored powder. Therefore, the resulting streak appears white or transparent, blending in with the streak plate. This confirms that the streak test is not the best way to distinguish peridot from other colorless-streaked minerals.
Other Essential Identification Properties
Since the colorless streak is not definitive for peridot, other physical and optical properties must be examined for positive identification. Hardness is consistently measured at 6.5 to 7 on the Mohs scale. This moderate hardness makes peridot durable enough for jewelry but also means it can be scratched by harder common gems like quartz.
Peridot crystallizes in the orthorhombic system, which is a significant factor in its optical behavior. The internal arrangement of atoms causes the stone to exhibit birefringence, which is a difference between the highest and lowest refractive indices. Peridot has a relatively high birefringence, typically ranging from 0.035 to 0.038.
This high birefringence results in a phenomenon known as double refraction. When viewing a faceted peridot, the rear facet edges appear visibly doubled when looking through the table of the stone. This characteristic doubling effect is a reliable and distinguishing feature of peridot that is rarely matched by similar-looking green gemstones.
Another important characteristic is the specific gravity, which is a measure of the mineral’s density. Peridot is relatively dense, with a specific gravity ranging from approximately 3.27 to 3.38. This density is noticeably higher than many other green gemstones. These combined properties are necessary to confirm a specimen’s identity when the streak test is inconclusive.