Optical calcite, known as Iceland spar, is a high-purity, transparent variety of calcite that exhibits strong double refraction. When a single line or image is viewed through a clean rhombohedral cleavage piece, it appears doubled, making it a classic specimen for physical optics study. Collectors should look for large, clear, and perfectly shaped rhombohedrons free of internal clouding or heavy twinning.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralcommonCalcite family

Optical Calcite

Also: Iceland Spar

Hardness: 3
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this optical calcite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch optical calcite with a known reference. Optical Calcite sits at Mohs 3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Optical Calcite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Optical Calcite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white.
5
Look at form & habit
Crystal system: trigonal. Typical habit: rhombohedral crystals.

Often confused with

Optical Calcite vs. its common look-alikes, and how to tell them apart in the field.

mineral

Gypsum

Mohs 2

How to tell apart: Optical Calcite is noticeably harder (Mohs 3 vs. 2).

How to tell apart: Quartz is the harder of the two (Mohs 7 vs. 3).

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Often found alongside optical calcite

Minerals reported to co-occur with optical calcite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: CaCO₃
Mohs hardness: 3
Density: 2.71 g/cm³
Colors: Colorless White
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Trigonal
Crystal habit: Rhombohedral Crystals
Cleavage: Perfect Rhombohedral in 3 Directions
Fluorescence: Often Fluorescent Under UV
Rarity: Common
Uses: Collector, Scientific, Optical
Host rock: Sedimentary Environments and Hydrothermal Veins
Typical price: $10-100 per specimen

Where rockhounds find optical calcite

Classic worldwide localities

Helgustadir, Iceland
Chihuahua, Mexico
Sonora, Mexico
Tennessee, USA

Field-hunting tip

Look in sedimentary environments and hydrothermal veins country, the host setting where optical calcite typically forms. If you start seeing quartz, fluorite, dolomite in float, you are in the right ground. Field specimens usually show a rhombohedral crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify optical calcite?+

Mohs hardness is 3. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white.

Where is optical calcite found?+

Notable localities include Helgustadir, Iceland; Chihuahua, Mexico; Sonora, Mexico; Tennessee, USA.

How much is optical calcite worth?+

Typical asking prices fall in the range of $10-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like optical calcite?+

Optical Calcite is most often confused with Gypsum, Halite, Quartz. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with optical calcite?+

Optical Calcite commonly co-occurs with Quartz, Fluorite, Dolomite, Barite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does optical calcite form in?+

Optical Calcite typically forms in sedimentary environments and hydrothermal veins. Working float back to the host body is the standard way to chase a fresh occurrence.

What is optical calcite used for?+

Optical Calcite is used in collector, scientific, optical.

Find optical calcite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.