Eastonite is a rare magnesium-rich mica belonging to the biotite group, often appearing as platy or foliated dark-colored crystals. It is predominantly found in contact-metamorphosed limestone or marble environments. Collectors prize it for its specific chemical composition which distinguishes it from more common phlogopite specimens.

Hardness

2.5-3

Mohs

Luster

Pearly

Streak

White

Transparency

Translucent

mineralrareBiotite Group family

Eastonite

Hardness: 2.5-3
Luster: Pearly
Streak: White
Transparency: Translucent

Is this eastonite?

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 eastonite with a known reference. Eastonite sits at Mohs 2.5-3, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Eastonite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Eastonite typically shows a pearly luster.
4
Match the color range
Compare against the expected color range: dark brown, black, greenish-brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: platy crystals, foliated masses, micaceous aggregates.

Often confused with

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

Often found alongside eastonite

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

All properties

Chemical formula: KMg₃AlSi₃O₁₀(OH)₂
Mohs hardness: 2.5-3
Density: 2.9-3.2 g/cm³
Colors: Dark Brown Black Greenish-brown
Streak: White
Luster: Pearly
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Platy Crystals, Foliated Masses, Micaceous Aggregates
Cleavage: Perfect Basal
Rarity: Rare
Uses: Collector, Research
Host rock: Metamorphosed Limestones and Marbles
Typical price: $10-50 per specimen

Where rockhounds find eastonite

Classic worldwide localities

Monroe, New York, USA
Grenville Province, Canada
Kola Peninsula, Russia

Field-hunting tip

Look in metamorphosed limestones and marbles country, the host setting where eastonite typically forms. If you start seeing calcite, diopside, spinel in float, you are in the right ground. Field specimens usually show a platy crystals, foliated masses, micaceous aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify eastonite?+

Mohs hardness is 2.5-3. It typically shows a pearly luster. The streak is white. Common colors include dark brown, black, greenish-brown.

Where is eastonite found?+

Notable localities include Monroe, New York, USA; Grenville Province, Canada; Kola Peninsula, Russia.

How much is eastonite worth?+

Typical asking prices fall in the range of $10-50 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like eastonite?+

Eastonite is most often confused with Phlogopite, Biotite, Annite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with eastonite?+

Eastonite commonly co-occurs with Calcite, Diopside, Spinel, Serpentine. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does eastonite form in?+

Eastonite typically forms in metamorphosed limestones and marbles. Working float back to the host body is the standard way to chase a fresh occurrence.

What is eastonite used for?+

Eastonite is used in collector, research.

Find eastonite on the map

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