Iceland Spar Identification Guide
A field guide to Iceland spar, the optically clear calcite famous for double refraction, with the rhomb, hardness, and acid tests.
Read the full Iceland Spar encyclopedia entry →
What Iceland Spar Looks Like
Iceland spar is a transparent, optical-grade variety of calcite (CaCO3). It is the classic mineral for demonstrating double refraction:
- Color: colorless and water-clear; sometimes faint yellow or gray
- Luster: vitreous
- Transparency: transparent, glass-clear
- Habit: cleaves into perfect rhombohedrons (leaning, slanted "boxes"); original crystals may be scalenohedral
Step-by-Step Field ID Checklist
- Look for the rhomb shape. A clear, slanted, parallelogram-faced block is the signature.
- Do the double-image test. Place the clear rhomb over a line or dot — you should see two separated images that move apart as you rotate the stone. This birefringence is diagnostic.
- Test hardness. Mohs 3; a copper coin or steel knife scratches it easily, and a fingernail will not.
- Check the cleavage. It splits cleanly in three directions into more rhombs.
- Run the acid test (see below).
Key Diagnostic Tests
- Double refraction: strong birefringence produces the famous double image — the best single test.
- Acid: fizzes vigorously in dilute hydrochloric acid (releases CO2). Even a drop of vinegar bubbles slowly.
- Hardness: 3 (soft).
- Cleavage: perfect rhombohedral in three directions.
- Streak: white.
- Density: ~2.71 g/cm3.
Common Look-Alikes
- Clear quartz: much harder (7), no acid fizz, does not cleave into rhombs, and shows no strong double image.
- Glass: harder, no cleavage, no fizz, single image.
- Halite (rock salt): cleaves into cubes (90°), tastes salty, and dissolves in water; calcite rhombs are slanted and do not taste salty.
- Gypsum (selenite): softer (2), cleaves into sheets/flexible flakes, no acid fizz.
- Topaz/danburite: far harder; no double image of this strength and no acid reaction.
Where It Is Found
The original and namesake source is Helgustadir in Iceland, historically the finest optical calcite. Excellent material also comes from Mexico (Chihuahua), the United States, Brazil, China, and other calcite-rich hydrothermal and sedimentary settings. It occurs in cavities, veins, and basalt vesicles.
Frequently asked questions
How can you tell if it's real Iceland spar?
Place the clear rhomb over a line: real Iceland spar splits it into two separate images (double refraction). It also has a hardness of only 3, cleaves into slanted rhombs in three directions, and fizzes in dilute acid or vinegar.
Why does Iceland spar show a double image?
Calcite is strongly birefringent, meaning light entering the crystal splits into two rays traveling at different speeds and angles, so a single line or dot viewed through it appears doubled.
What does Iceland spar look like?
It looks like a clear, colorless glass block shaped like a leaning parallelogram (rhombohedron) with smooth, shiny cleavage faces.
Iceland spar vs quartz: how are they different?
Iceland spar is calcite — soft (hardness 3), fizzes in acid, cleaves into rhombs, and shows a strong double image. Quartz is hard (7), does not fizz, has no cleavage, and shows no strong double refraction.
Iceland Spar identified by the community
Recent Iceland Spar specimens identified with Rock Identifier.
