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 — that is 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.