Eglestonite is a rare mercury oxychloride mineral that is most famous for its occurrence in the Terlingua mining district of Texas. It typically presents as small, brilliant yellow-orange or yellow-brown crystals that quickly darken to a dull brownish-black upon exposure to light, making fresh, bright specimens highly sought after by collectors.

Hardness

2.5

Mohs

Luster

Adamantine

Streak

Yellowish-white

Transparency

Translucent

mineralrare

Eglestonite

Hardness: 2.5
Luster: Adamantine
Streak: Yellowish-white
Transparency: Translucent

Is this eglestonite?

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 eglestonite with a known reference. Eglestonite sits at Mohs 2.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Eglestonite leaves a yellowish-white streak.
3
Read the luster
Hold the specimen under a strong light. Eglestonite typically shows a adamantine luster.
4
Match the color range
Compare against the expected color range: yellow, yellow-brown, orange, brown.
5
Look at form & habit
Crystal system: cubic. Typical habit: dodecahedral crystals, massive, crusts.

Often confused with

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

mineral

Calomel

Mohs 1.5-2

How to tell apart: Streak differs — Eglestonite leaves yellowish-white, Calomel leaves white.

How to tell apart: Streak differs — Eglestonite leaves yellowish-white, Terlinguaite leaves yellow.

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Often found alongside eglestonite

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

All properties

Chemical formula: Hg₄Cl₂O
Mohs hardness: 2.5
Density: 8.32 g/cm³
Colors: Yellow Yellow-brown Orange Brown
Streak: Yellowish-white
Luster: Adamantine
Transparency: Translucent
Crystal system: Cubic
Crystal habit: Dodecahedral Crystals, Massive, Crusts
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Mercury-rich Hydrothermal Deposits
Typical price: $50-500 thumbnail

Where rockhounds find eglestonite

Classic worldwide localities

Terlingua, Texas, USA
Idrija, Slovenia
Almadén, Spain
Mount Diablo, California, USA

Field-hunting tip

Look in mercury-rich hydrothermal deposits country — that is the host setting where eglestonite typically forms. If you start seeing calomel, terlinguaite, cinnabar in float, you are in the right ground. Field specimens usually show a dodecahedral crystals, massive, crusts habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify eglestonite?+

Mohs hardness is 2.5. It typically shows a adamantine luster. The streak is yellowish-white. Common colors include yellow, yellow-brown, orange, brown.

Where is eglestonite found?+

Notable localities include Terlingua, Texas, USA; Idrija, Slovenia; Almadén, Spain; Mount Diablo, California, USA.

How much is eglestonite worth?+

Typical asking prices fall in the range of $50-500 thumbnail. Quality, size, and provenance can move individual specimens well outside that range.

Is eglestonite safe to handle?+

It contains toxic constituents. Contains mercury and is highly toxic. Handle with extreme care; use gloves, wash hands thoroughly after contact, and store in a sealed container to prevent ingestion or inhalation of dust. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like eglestonite?+

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

What minerals are found with eglestonite?+

Eglestonite commonly co-occurs with Calomel, Terlinguaite, Cinnabar, Mercury. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does eglestonite form in?+

Eglestonite typically forms in mercury-rich hydrothermal deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is eglestonite used for?+

Eglestonite is used in collector.

Find eglestonite on the map

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