Nephrite is one of the two mineral materials, along with jadeite, collectively known as jade. While jadeite is often associated with the most vivid, emerald-like green, nephrite is the variety historically revered across many cultures, particularly in China. Nephrite is recognized for a broad spectrum of green hues, ranging from pale, watery shades to colors so dark they appear almost black. Its natural palette also includes highly valued whites and a range of secondary tones like yellow, brown, and gray.
The Spectrum of Green and White Nephrite
Nephrite’s most celebrated colors fall into the green and white spectrum, often described using specific trade names. Dark greens are called “spinach jade” due to their rich, deep coloration, sometimes containing small black inclusions. Other desirable shades include “moss green” and “olive,” which feature varying saturation and a characteristic waxy luster. These colors are prized when they are evenly distributed and translucent.
The white variety is the other highly valued end of the spectrum, most famously termed “mutton fat” jade. This name describes its creamy, translucent white to very pale yellow appearance, resembling rendered animal fat. Historically, “mutton fat” jade is the most esteemed nephrite, judged on color purity, fine texture, and greasy sheen. Pure, iron-free nephrite is white; the slightest introduction of coloring elements shifts the stone into the green or cream range.
How Impurities Create Specific Hues
Nephrite is a variety of the actinolite-tremolite mineral series, a calcium-magnesium-iron silicate amphibole. The fundamental cause of nephrite’s color is the varying ratio of magnesium to iron within its chemical structure. The mineral tremolite is the magnesium-rich end-member, while actinolite is the iron-rich end-member. Color changes occur specifically when iron ions replace magnesium ions in the crystal lattice.
As iron replaces magnesium in the crystal lattice, the mineral shifts toward the actinolite end-member, causing the color to darken to green. Low iron concentrations produce pale green and yellowish-green hues. Conversely, high concentrations result in deep “spinach” greens and very dark, near-black varieties.
Bright, saturated green hues are also influenced by other trace elements, such as chromium (Cr). Chromium ions contribute to vibrant color in some high-quality green nephrite, particularly those from Russian or Siberian deposits. These elements modify how the mineral absorbs light, leading to diverse and specific shades.
Secondary Colors and Tones
Black nephrite, sometimes called “black jade,” owes its appearance to two main factors. The color results either from extremely high concentrations of iron, pushing the hue past deep green, or from the presence of dense inclusions. These inclusions are often fine grains of graphite or magnetite distributed throughout the stone’s fibrous structure.
Yellowish-brown and reddish hues result from secondary geological processes rather than internal trace elements. These colors form when iron present in the nephrite oxidizes due to long-term environmental exposure, similar to rust. This surface alteration creates a patina of reddish-brown, yellow, or orange tones, often seen in ancient carved pieces.
Oxidation Products
The iron oxidation creates secondary mineral products like hematite (reddish-brown) and goethite (yellow-brown). These products permeate the micro-fissures near the stone’s surface.
Gray Nephrite
Gray nephrite is another common secondary color. It often represents a low concentration of iron insufficient to produce a distinct green. Alternatively, gray can result from a mixture of the white tremolite and dark actinolite components.