Feruvite Identification Guide

What Feruvite Looks Like

Feruvite is a rare calcium-iron-rich member of the tourmaline group (the calcic analogue of schorl). Like all tourmalines it forms trigonal prismatic crystals, typically dark brown to brownish-black, with the strong vitreous luster and three-sided cross-section that mark the group.

• Color: dark brown to brownish-black, sometimes greenish-brown
• Luster: vitreous (glassy)
• Transparency: translucent on thin edges to opaque
• Habit: elongate prisms with a rounded-triangular (trigonal) cross-section and lengthwise striations

Step-by-Step Field ID Checklist

1. Look for the tourmaline prism shape — long crystals with a distinctive rounded-triangular cross-section.
2. Check for vertical striations running the length of the crystal — a hallmark of the tourmaline group.
3. Note the dark brown-black color rather than the deep black of schorl.
4. Test hardness — it should scratch glass and quartz readily.
5. Confirm no cleavage — tourmalines fracture rather than cleave.

Key Diagnostic Tests

• Mohs hardness 7–7.5: Scratches glass and quartz; resists a steel knife. This rules out softer dark prismatic minerals like hornblende (5–6).
• Cleavage: None to indistinct — tourmaline breaks with uneven to conchoidal fracture, unlike amphiboles which have two good cleavages.
• Cross-section: The rounded triangular outline is diagnostic of tourmaline.
• Streak: White to pale grey-brown.
• Density: ~3.2 g/cm³.
• Note: Distinguishing feruvite from schorl or dravite by eye alone is unreliable; precise identification requires chemical or instrumental analysis because all are dark tourmalines.

Common Look-Alikes and How to Tell Them Apart

• Schorl (iron tourmaline): Schorl is sodium-dominant and usually jet black; feruvite is calcium-dominant and tends toward dark brown. Field separation is difficult — chemistry confirms it.
• Dravite (magnesium tourmaline): Brown like feruvite, but magnesium-rich rather than iron-calcium-rich. Composition, not appearance, is the true distinction.
• Hornblende / dark amphiboles: Softer (Mohs 5–6) and show two good cleavages at ~120°; feruvite is harder with no cleavage and a triangular cross-section.
• Black andradite/melanite garnet: Garnet forms equant dodecahedral crystals, not elongate striated prisms.

Where It Is Typically Found

Feruvite occurs in metamorphosed, boron-bearing calcium-rich rocks and certain skarns and contact zones. The type locality is Cuvier Island, New Zealand, and it has been reported from scattered metamorphic and hydrothermal settings worldwide. As a rare species it is mostly of interest to collectors and is typically confirmed by laboratory analysis rather than field tests alone.