What makes it “Iceland spar”?

It’s calcite—same chemistry as countless marbles—but unusually clear, colorless, and strain‑free. Those qualities let light split neatly into two rays inside the crystal, producing a crisp double image. Optically, calcite is uniaxial negative with a very high birefringence (n_ω ≈ 1.658, n_ε ≈ 1.486).

Names you’ll hear

• Iceland spar — historic trade name for optical‑grade calcite.
• Optical calcite — descriptive name used in labs and teaching sets.
• Sunstone (legend) — medieval texts mention a “sunstone”; some researchers propose calcite could help find the sun via polarized skylight. The romance is strong; the scholarship is still debated.

Hydrothermal veins & cavities

Clear calcite precipitates from carbonate‑rich fluids moving through fractures and vugs. Slow, steady growth and low impurity levels give us those water‑clear rhombs.

From limestone to spar

In limestones and marbles, recrystallization can grow larger calcite crystals. Most are milky; a lucky few emerge as optical grade.

Why Iceland?

Historic lenses and prisms came from the Helgustaðir quarry in East Iceland, famed for exceptionally pure crystals. Today, optical‑clear calcite also comes from Mexico, China, and other carbonate provinces.

Palette & luster

• Colorless to water‑clear — sometimes faint honey edges.
• Frosted bevels — from natural growth or gentle wear.
• Warm glow — along very thin edges in transmitted light.

Luster is vitreous on fresh faces and pearly on cleavage surfaces—soft and elegant.

Habit words

• Rhombohedra — the classic box‑that‑isn’t‑a‑box shape.
• Twins & lamellae — subtle internal banding under certain lights.
• Phantom growth — occasional faint internal outlines of earlier stages.

Photo tip: Use soft, raking light from one side to catch pearly cleavage without flooding the crystal. A dark matte base makes the double image easier to show.

Double‑image test

Place the crystal over small print. You’ll see two crisp images. Rotate the rhomb and watch them separate or merge—an instant birefringence demo.

Cleavage clues

Look for perfect, mirror‑flat cleavage planes meeting at rhombohedral angles. Minute knocks often step these planes like tiny terraces.

Growth zoning

Under side lighting, faint internal lamellae or “phantom” outlines may appear. They don’t spoil clarity; they prove the crystal’s slow, orderly growth.

Quartz (rock crystal)

Harder (Mohs 7), no double image, hexagonal habit, and no acid fizz. If it doubles text, it’s not quartz.

Aragonite

Same chemistry, different structure (orthorhombic). Typically less clear as large single rhombs and has different twinning habits.

Halite & selenite

Halite tastes salty (please don’t taste specimens) and forms cubes; selenite (gypsum) is softer (Mohs 2), with fibrous cleavage and silky sheen.

Glass

Amorphous, no birefringence, often with rounded bubbles; lacks perfect cleavage and acid reaction.

Quick checklist

• Instant double refraction on text?
• Effervescence with dilute acid?
• Soft (scratches with copper coin) & perfect rhombohedral cleavage? → Iceland spar.

Where it shines

Historic crystals came from Helgustaðir in East Iceland. Modern optical‑clear calcite is also found in parts of Mexico (Chihuahua), China, Brazil, the USA, and Norway. Many localities produce calcite; only a few gift us true spar.

How people used it

From early Nicol prisms and polarizing microscopes to classroom optics kits, Iceland spar has been science’s see‑through sidekick. It’s also a favorite for display because visitors can do an experiment with it in seconds.

Handling

• Hold over a soft surface; perfect cleavage means drops can split corners.
• Dust with a puffer or very soft brush; avoid gritty cloths that can haze the faces.
• Keep dry and away from acids (even vinegar will fizz).

Mounting & storage

• Seat on neutral foam or an acrylic stand with gentle support.
• Do not wedge tight prongs against edges; let the rhomb “breathe.”
• A clear cover keeps fingerprints and dust at bay without muting sparkle.

Display & photography

• Place over a small sample of text for instant interactivity.
• Use a single soft key light plus a dark matte plinth to show the double image cleanly.
• Backlight thin edges lightly for a honey rim without washing clarity.

Double‑print trick

Set the rhomb on a word like “light.” Rotate the crystal slowly: the two “lights” move relative to each other and then reunite. That’s birefringence you can feel with your eyes.

Fizz & fix

On a scrap piece (not your display gem), a drop of dilute vinegar produces CO₂ fizz—classic calcite behavior. Neutralize and rinse immediately if you try this at home.