Anorthosite is a phaneritic, intrusive igneous rock composed almost entirely of plagioclase feldspar, typically containing 90% or more of this mineral. It is often gray to white in color and is notable for forming large plutonic bodies on Earth as well as making up the bulk of the lunar highlands. Collectors typically look for variations containing accessory minerals like ilmenite or labradorite with iridescent optical properties.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Opaque

igneouscommon

Anorthosite

Hardness: 6
Luster: Vitreous
Streak: White
Transparency: Opaque

Is this anorthosite?

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 anorthosite with a known reference. Anorthosite sits at Mohs 6 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Anorthosite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Anorthosite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, gray, bluish-gray, black.
5
Look at form & habit
Typical habit: massive.

Often confused with

Anorthosite vs. its common look-alikes — and how to tell them apart in the field.

igneous

Gabbro

Mohs 6-7

How to tell apart: Streak differs — Anorthosite leaves white, Gabbro leaves none.

Often found alongside anorthosite

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

All properties

Mohs hardness: 6
Density: 2.6-2.9 g/cm³
Colors: White Gray Bluish-gray Black
Streak: White
Luster: Vitreous
Transparency: Opaque
Crystal habit: Massive
Rarity: Common
Uses: Building Stone, Decorative Stone, Collector
Host rock: Plutonic Igneous Rocks
Typical price: $5-50 for hand specimens

Where rockhounds find anorthosite

Classic worldwide localities

Adirondack Mountains, USA
Labrador, Canada
Rogaland, Norway
Madagascar
Moon (Lunar Highlands)

Field-hunting tip

Look in plutonic igneous rocks country — that is the host setting where anorthosite typically forms. If you start seeing plagioclase, ilmenite, magnetite in float, you are in the right ground. Field specimens usually show a massive habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify anorthosite?+

Mohs hardness is 6. It typically shows a vitreous luster. The streak is white. Common colors include white, gray, bluish-gray, black.

Where is anorthosite found?+

Notable localities include Adirondack Mountains, USA; Labrador, Canada; Rogaland, Norway; Madagascar; Moon (Lunar Highlands).

How much is anorthosite worth?+

Typical asking prices fall in the range of $5-50 for hand specimens. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like anorthosite?+

Anorthosite is most often confused with Gabbro, Diorite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with anorthosite?+

Anorthosite commonly co-occurs with Plagioclase, Ilmenite, Magnetite, Pyroxene. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does anorthosite form in?+

Anorthosite typically forms in plutonic igneous rocks. Working float back to the host body is the standard way to chase a fresh occurrence.

What is anorthosite used for?+

Anorthosite is used in building stone, decorative stone, collector.

Find anorthosite on the map

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