Schorl Identification Guide

What Schorl Looks Like

Schorl is the iron-rich, black variety of tourmaline, by far the most common tourmaline species. It is opaque black (occasionally very dark blue-black or brownish-black) with a vitreous to submetallic luster. Its most diagnostic feature is its crystal form: long, slender prismatic crystals with heavy vertical striations running their length and a distinctive rounded-triangular cross-section (a curved-sided triangle). Crystals often occur as sprays, radiating clusters, or single columns, and may be doubly terminated. Schorl is frequently found embedded in quartz, granite, and pegmatite, and a popular gem material is "tourmalinated quartz" — clear quartz threaded with black schorl needles.

Step-by-Step Field Checklist

1. Look at cross-section. The hallmark is a rounded-triangular (curved-triangle) cross-section — unique among common black prismatic minerals.
2. Check striations. Strong parallel grooves run lengthwise along the prism faces.
3. Confirm color and luster. Deep black with a glassy to slightly metallic shine.
4. Streak it. Schorl gives a colorless to pale gray-brown streak, never the black/dark-red of iron ore minerals.
5. Test hardness. Tourmaline is Mohs 7–7.5; it scratches glass and steel will not scratch it.
6. Check magnetism and lack of cleavage. It is essentially non-magnetic and shows no cleavage, breaking with uneven to conchoidal fracture.

Key Diagnostic Tests

• Hardness: 7–7.5 — scratches glass and steel.
• Cross-section/striations: rounded-triangular prism with lengthwise striations — diagnostic.
• Cleavage: none (indistinct); uneven to conchoidal fracture.
• Streak: colorless to pale gray.
• Magnetism: non-magnetic.
• Specific gravity: ~3.1–3.3; pyroelectric/piezoelectric when warmed or stressed.

Common Look-Alikes and How to Tell Them Apart

• Hornblende/black amphibole: softer (Mohs 5–6), has two cleavages meeting at ~120 degrees, and a diamond-shaped cross-section, whereas schorl has no cleavage and a triangular cross-section.
• Black rutile: adamantine luster, square cross-section, and a pale brown streak; rutile is softer (6–6.5) and denser.
• Magnetite/ilmenite: metallic, magnetic (magnetite strongly so), with black streak; schorl is non-magnetic with a pale streak.
• Augite/pyroxene: softer with stubby crystals and ~90-degree cleavage.
• Obsidian/black glass: conchoidal fracture but no crystal form, no striations, and softer (~5–5.5).

Where Schorl Is Found

Schorl is an abundant accessory mineral in granites, pegmatites, and metamorphic schists and gneisses, and concentrates in placer gravels because it is hard and durable. Outstanding crystals come from pegmatites in Brazil, Pakistan, Namibia, Madagascar, and the United States (Maine, California, Colorado). It commonly grows alongside quartz, feldspar, and mica, and well-formed striated black prisms in pegmatite pockets are a classic field find.