Chrome Tourmaline Identification Guide

What Chrome Tourmaline Looks Like

Chrome tourmaline is a vivid emerald to grassy green tourmaline colored by chromium and vanadium (chemically usually a dravite or elbaite/dravite intermediate). The green is rich and saturated—closer to emerald than the bluish or yellowish greens of ordinary green tourmaline (verdelite). Luster is vitreous, and quality material is transparent. Crystals show the classic tourmaline form: elongated prisms with a rounded-triangular cross-section and strong lengthwise striations.

Step-by-Step Field ID Checklist

1. Note the color. A pure, saturated chromium green that often looks "emerald-like."
2. Check the cross-section. Rounded-triangular prisms are diagnostic of tourmaline.
3. Look for striations. Deep parallel grooves down the prism faces.
4. Observe pleochroism. Rotate a transparent piece: chrome tourmaline shows strong green/darker-green dichroism. It is usually cut with the table parallel to the c-axis to lighten the dark direction.
5. Test hardness. Mohs 7-7.5.
6. Check cleavage. None — conchoidal/uneven fracture.

Key Diagnostic Tests

• Hardness: 7-7.5 (scratches glass and quartz).
• Cleavage: Absent; brittle, conchoidal fracture.
• Streak: White.
• Density: ~3.0-3.1 g/cm3.
• Magnetism/acid: Non-magnetic; inert to acid.
• Optics: Strongly dichroic; doubly refractive; chromium causes a red reaction through a Chelsea filter and red flashes under strong light. Piezo-/pyroelectric (attracts dust when warmed).

Common Look-Alikes and How to Tell Them Apart

• Emerald (beryl): Harder (7.5-8), lower density (~2.7), hexagonal not triangular, often heavily included. Emerald shows weaker doubling; tourmaline shows stronger birefringence (doubled back facets).
• Tsavorite / chrome diopside: Tsavorite is isometric garnet (no pleochroism, no striations, denser ~3.6); chrome diopside is softer (5.5-6.5) with right-angle cleavage.
• Green tourmaline (verdelite): Same family but iron-colored, usually less pure green and lacking the red Chelsea-filter reaction.
• Green glass / doublets: No striations, no pleochroism, bubbles present.

Where It Is Typically Found

The premier source of chrome tourmaline is Tanzania, especially the Landanai, Umba, and Commander areas near the chromium-rich ultramafic belts; nearby Kenya also produces it. These tourmalines form where boron-rich pegmatitic or metamorphic fluids interact with chromium-bearing host rocks. Smaller amounts come from other East African localities.

Field Tips and Common Mistakes

Because chrome tourmaline is the green tourmaline most often confused with emerald, lean on three quick separators: the rounded-triangular cross-section and lengthwise striations (emerald is hexagonal), the stronger doubling of back facet edges seen through the table, and the red reaction under a Chelsea filter shared with emerald but not with iron-colored verdelite. Pleochroism is your friend: rotate the stone and you should see two distinct greens, with one direction noticeably darker. Cutters orient the table parallel to the long axis to keep the darker pleochroic direction from dominating, so a well-cut stone reads brighter face-up than its rough suggests. Do not confuse it with tsavorite garnet, which is singly refractive and non-pleochroic. Finally, beware glass and doublets in commercial parcels: glass lacks striations and pleochroism and often shows bubbles, while a doublet may reveal a join line and color confined to a thin layer.