Alexandrite Identification Guide

What Alexandrite Looks Like

Alexandrite is the rare, color-change variety of chrysoberyl (BeAl2O4), colored by chromium. Its signature is a striking color change with the light source.

• Color: green to bluish-green in daylight/fluorescent light, shifting to red, purplish-red, or raspberry under incandescent (warm) light — the famous 'emerald by day, ruby by night' effect.
• Luster: vitreous, bright.
• Transparency: transparent to translucent.
• Habit: typically as tabular or prismatic crystals and, very characteristically, cyclic trillings (pseudohexagonal twinned crystals with re-entrant notches). Cat's-eye (chatoyant) alexandrite also occurs.

Step-by-Step Field ID Checklist

1. Do the color-change test. View the stone in daylight, then under a warm incandescent bulb. A clear green-to-red shift is the defining trait.
2. Check pleochroism. Alexandrite is strongly pleochroic — green, orange-yellow, and red appear along different crystal directions; turn the stone and watch the colors change with viewing angle (distinct from the lighting change).
3. Test hardness. Mohs 8.5 — extremely hard; it scratches topaz and quartz easily.
4. Look for twinning. Pseudohexagonal trilling crystals with notches strongly suggest chrysoberyl.
5. Check density. High SG (~3.7), heavy for its size.
6. Streak. White.

Key Diagnostic Tests

• Hardness: 8.5 — third only to diamond and corundum among common gems.
• Color change: the diagnostic property; genuine alexandrite changes hue with light source.
• Pleochroism: strong trichroism, separate from the lighting color change.
• Specific gravity: ~3.70–3.75.
• Refractive index: high (~1.74–1.75), giving bright luster.
• Cleavage: distinct in one direction but poor overall; conchoidal fracture.

Common Look-Alikes and How to Tell Them Apart

• Color-change garnet: also shifts color, but garnet is softer (~7–7.5), singly refractive (no pleochroism), and has different SG/RI. Alexandrite's strong pleochroism and 8.5 hardness separate it.
• Color-change sapphire: harder distinction; sapphire is harder (9) and corundum, with different RI and dichroism. Lab RI/SG settles it.
• Synthetic alexandrite and color-change synthetic corundum/spinel: very common imitations; curved growth lines, gas bubbles, or an over-perfect color change suggest synthetic. The classic 'alexandrite simulant' is color-change synthetic corundum (often with a purple/mauve shift) — its change is less green-to-red and more blue-to-purple.
• Andesine/tourmaline: rarely confused; lower hardness and weaker change.

Where Alexandrite Is Found

Alexandrite was first found in the Ural Mountains of Russia (in mica schists associated with emerald). Major modern sources are Brazil (Hematita, Minas Gerais), Sri Lanka, and East Africa (Tanzania, the Lake Manyara area, and others). It forms where beryllium-bearing rocks meet chromium-rich rocks, such as pegmatites intruding ultramafic/mica-schist host rocks. Look for it with chrysoberyl, emerald, and phenakite in such metamorphic-pegmatitic settings.