Chrome-Dravite
NaMg3Cr6(Si6O18)(BO3)3(OH)3(OH)
A chromium-dominant tourmaline related to dravite, producing intensely deep green to blackish-green crystals from chromium-rich metamorphic rocks.
- Mohs hardness
- 7-7.5
- Color
- Deep green to blackish-green
- Type
- mineral
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Overview
Chrome-dravite (chromdravite) is a chromium-dominant member of the tourmaline supergroup in which chromium replaces the aluminum normally found in dravite's octahedral sites. The high chromium content gives extremely saturated dark green to blackish-green crystals.
It is the chromium analogue of dravite and was described from Russian localities. As a species it is rare, but chromium-bearing dravites grading toward this composition produce some of the richest green tourmalines known.
It is valued by mineral collectors and, where transparent material exists, contributes to the vivid "chrome" green seen in gem tourmaline.
Formation & geology
Chrome-dravite forms in chromium-rich metamorphic and metasomatic environments, often associated with rocks derived from ultramafic protoliths or chromium-bearing sediments, where boron and magnesium are also available.
Its type locality lies in Karelia, Russia (the Onega/Lake region), in chromium- and boron-enriched metamorphic rocks. Chromium-rich dravites also occur in East African deposits associated with metamorphosed ultramafic and graphitic rocks.
The key requirement is a source of chromium combined with the magnesium-rich, boron-bearing fluids that favor the dravite structure during metamorphism.
How to identify it
Chrome-dravite forms deep green to nearly black-green prismatic tourmaline crystals with striated faces, triangular cross-section, vitreous luster, hardness 7-7.5, and no cleavage. Thin sections or small grains can show strong green color and pleochroism.
Distinguishing true chrome-dravite from chromium-rich dravite or chrome/vanadium-colored elbaite requires chemical analysis. The intensity of green and association with chromium-bearing rocks are practical clues.
Look-alikes include verdelite (green elbaite), chrome diopside, and emerald; tourmaline's striations, habit, and optics distinguish it.
Uses & significance
Chrome-dravite itself is largely a specimen and research mineral, but the chromium chemistry it embodies is responsible for prized vivid-green gem tourmalines ("chrome tourmaline") cut from related material.
Fine green tourmaline of chrome composition can be valuable in jewelry when transparent. As crystals, chrome-dravite is collected for systematic mineralogy and studied as evidence of chromium mobility in metamorphic systems. Metaphysically, green tourmalines are associated with the heart and vitality.
Frequently asked questions
What makes chrome-dravite green?
Chromium occupying aluminum sites in the dravite structure produces its intense green color.
How is chrome-dravite different from dravite?
Dravite is aluminum-dominant in those octahedral sites, while chrome-dravite is chromium-dominant, giving far more saturated green.
Where is chrome-dravite found?
Its type locality is in Karelia, Russia, and chromium-rich dravites also occur in East African metamorphic deposits.
Is chrome-dravite used as a gem?
The species is mostly a collector mineral, but related chromium-rich green tourmaline is cut as valuable chrome tourmaline.
Chrome-Dravite guides
In-depth guides for identifying, valuing, and understanding Chrome-Dravite.
