Hilgardite is a rare borate mineral primarily found in marine evaporite deposits. It is best known for its exceptional fluorescence under UV light and often occurs as small, sharp, wedge-shaped crystals in halite-rich environments.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Hilgardite

Hardness: 5
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this hilgardite?

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 hilgardite with a known reference. Hilgardite sits at Mohs 5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Hilgardite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Hilgardite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white, pink, yellowish.
5
Look at form & habit
Crystal system: triclinic. Typical habit: tabular crystals, wedge-shaped.

Often confused with

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

mineral

Danburite

Mohs 7-7.5

How to tell apart: Danburite is the harder of the two (Mohs 7-7.5 vs. 5).

How to tell apart: Boracite is the harder of the two (Mohs 7-7.5 vs. 5).

Compare →

Often found alongside hilgardite

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

All properties

Chemical formula: Ca₂B₅O₉Cl·H₂O
Mohs hardness: 5
Density: 2.71 g/cm³
Colors: Colorless White Pink Yellowish
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Triclinic
Crystal habit: Tabular Crystals, Wedge-shaped
Cleavage: Distinct On {001}
Fluorescence: Bright Yellow to Orange Under UV Light
Rarity: Rare
Uses: Collector
Host rock: Evaporite Deposits
Typical price: $20-150 per specimen

Where rockhounds find hilgardite

Classic worldwide localities

Haguenau, France
Winslow, Arizona
Windsor, Nova Scotia
Boulby Mine, United Kingdom

Field-hunting tip

Look in evaporite deposits country — that is the host setting where hilgardite typically forms. If you start seeing halite, sylvite, anhydrite in float, you are in the right ground. Field specimens usually show a tabular crystals, wedge-shaped habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify hilgardite?+

Mohs hardness is 5. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white, pink, yellowish.

Where is hilgardite found?+

Notable localities include Haguenau, France; Winslow, Arizona; Windsor, Nova Scotia; Boulby Mine, United Kingdom.

How much is hilgardite worth?+

Typical asking prices fall in the range of $20-150 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like hilgardite?+

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

What minerals are found with hilgardite?+

Hilgardite commonly co-occurs with Halite, Sylvite, Anhydrite, Boracite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does hilgardite form in?+

Hilgardite typically forms in evaporite deposits. Working float back to the host body is the standard way to chase a fresh occurrence.

What is hilgardite used for?+

Hilgardite is used in collector.

Find hilgardite on the map

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