Serpentine is the name for a large group of hydrated magnesium silicate minerals that form when magnesium-rich rocks undergo alteration underground. The name comes from the Latin word serpens, meaning “snake,” referencing the mottled, scaly, and patterned appearance common to many forms. Due to the variety in its chemical makeup and formation conditions, serpentine encompasses a wide spectrum of colors. These range from the most common greens to less frequent shades of yellow, brown, white, and black.
The Defining Shades of Green
The most characteristic and widespread color for serpentine is green, appearing in a diverse array of hues. Shades range from pale, translucent apple-green to deep, opaque forest-green or yellowish-green. This color is linked to the mineral’s core chemical structure, a hydrated magnesium silicate, which often incorporates minor amounts of iron.
The typical green color is often accompanied by a distinctive waxy, greasy, or silky surface quality. Serpentine rock, known as serpentinite, frequently displays this green material crisscrossed by veins or mottling of other colors. The intensity of the green varies based on the concentration of iron within the silicate lattice.
How Impurities Alter Serpentine’s Hue
While the base structure dictates the general green tone, the inclusion of trace elements significantly broadens the color palette. Deviations from standard green are caused by the substitution of elements like iron, nickel, and chromium for magnesium within the crystal lattice. Iron is the most impactful element, occurring in two oxidation states: ferrous (Fe2+) and ferric (Fe3+).
Ferrous iron (Fe2+) content is primarily responsible for the strength of the green hue. Conversely, the presence of ferric iron (Fe3+) tends to shift the color toward yellow or brown. High concentrations of iron can lead to very dark shades, resulting in reddish-brown, brown, or black serpentine. Additionally, iron from precursor rocks often forms tiny, dispersed inclusions of magnetite, which darkens the serpentine and sometimes imparts a subtle bluish cast.
Serpentine Varieties and Their Unique Colors
The serpentine mineral group is defined by three primary varieties: lizardite, chrysotile, and antigorite, each exhibiting a slightly different crystal structure and color range.
Lizardite
Lizardite is often the most abundant form and is commonly recognized by its fine-grained, platy structure. It typically results in colors ranging from pale green to white. Many ornamental serpentine stones prized for their mottled appearance are rich in lizardite.
Antigorite
Antigorite forms under higher temperatures and is typically a darker, more massive material. It is frequently found as dark green plates, but can also occur in shades of yellowish-green, gray, brown, or black. A harder variety, known as Bowenite, displays a translucent, apple-green to blue-green color and is often used in jewelry and carvings as a substitute for jade.
Chrysotile
Chrysotile is the fibrous variety of serpentine. It often appears as light gray, white, or golden-yellow fibers within veins of the more massive serpentine.