Diopside

Diopside is a rock-forming member of the clinopyroxene group, a calcium-magnesium silicate (CaMgSi2O6) crystallizing in the monoclinic system as short, stubby prisms with a near-square cross-section. Hardness is about 6, with two cleavage planes intersecting at 93/87 degrees, as is typical of all the pyroxenes. Color ranges from colorless and pale grey-green through grass-green, deep bottle-green, violet, and near-black, reflecting substitution of iron, chromium, or manganese for magnesium; it forms a complete chemical replacement series with hedenbergite, in which ferrous iron completely replaces magnesium in the structure. Geologically, diopside results from the thermal metamorphism of siliceous limestones or dolomites, and in calc-silicate skarns produced by contact metamorphism it is associated with wollastonite, vesuvianite, grossular, and tremolite. It also occurs in mantle-derived peridotites and kimberlites, where intensely green chromian varieties are easily spotted in the heavy mineral fraction of sand, making it an indicator mineral in diamond prospecting.

Classic collector localities span several continents. In the United States, DeKalb and the Calvin Mitchell farm in St. Lawrence County, New York, along with the Tilly Foster Iron Mine in Putnam County, are famous for producing large, gemmy, transparent green crystals prized in historic collections. In Europe, Ala and Mount Monzoni in Italy's Alpine belt yield fine skarn-hosted green diopside, while the manganese deposit of Praborna near Saint-Marcel, in Italy's Aosta Valley, is the type locality for the rare violet-blue variety violan. Slyudyanka, on the shores of Lake Baikal in Russia, produces both deep-green chromian crystals known locally as baikalite and violet lavrovite, and Outokumpu in Finland is another classic source of fine deep-green chrome-bearing crystals. Other notable occurrences include Nordmark and Långban in Sweden, and Franklin, New Jersey, both known for manganiferous varieties such as jeffersonite and schefferite.

For gem collectors, the most sought-after material is vivid chrome diopside from the Inagli deposit near Aldan in the Sakha Republic (Yakutia), Siberia, discovered near diamond-bearing kimberlite pipes and marketed since the late 1980s as "Siberian" or "Russian emerald" for its intense chromium-driven green. Additional chrome diopside sources include Pakistan, Myanmar, Madagascar, Tanzania, Kenya, and Finland, while Nammakal in India supplies chatoyant black star diopside showing a sharp four-rayed asterism from oriented magnetite needle inclusions. Collectors value diopside for this remarkable range of colors and habits within a single species — from museum-grade New York crystals to Siberian emerald-green gems and Alpine violan — combined with its role as a textbook skarn and mantle-indicator mineral for petrologists and diamond prospectors alike.