Harkerite is a rare borosilicate mineral found in high-temperature contact metamorphic zones. It typically forms white to colorless dodecahedral crystals that closely resemble common garnet, requiring laboratory analysis to confirm its unique composition.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineralrareGarnet Group family

Harkerite

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

Is this harkerite?

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 harkerite with a known reference. Harkerite 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. Harkerite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Harkerite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, colorless, gray.
5
Look at form & habit
Crystal system: trigonal. Typical habit: dodecahedral crystals, massive, granular.

Often confused with

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

mineral

Garnet

Mohs 6.5-7.5

How to tell apart: Garnet is the harder of the two (Mohs 6.5-7.5 vs. 5).

How to tell apart: Harkerite is noticeably harder (Mohs 5 vs. 3).

Often found alongside harkerite

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

All properties

Chemical formula: Ca₄Mg₂Al(Si₃Al)O₁₂(BO₃)CO₃
Mohs hardness: 5
Density: 2.7-2.8 g/cm³
Colors: White Colorless Gray
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Trigonal
Crystal habit: Dodecahedral Crystals, Massive, Granular
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Contact Metamorphic Limestone (skarns)
Typical price: $20-100 thumbnail

Where rockhounds find harkerite

Classic worldwide localities

Skye, Scotland
Monte Somma, Italy
Chichibu, Japan
Brooks Mountain, Alaska, USA

Field-hunting tip

Look in contact metamorphic limestone (skarns) country — that is the host setting where harkerite typically forms. If you start seeing calcite, gehlenite, spinel in float, you are in the right ground. Field specimens usually show a dodecahedral crystals, massive, granular habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify harkerite?+

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

Where is harkerite found?+

Notable localities include Skye, Scotland; Monte Somma, Italy; Chichibu, Japan; Brooks Mountain, Alaska, USA.

How much is harkerite worth?+

Typical asking prices fall in the range of $20-100 thumbnail. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like harkerite?+

Harkerite is most often confused with Garnet, Calcite, Scapolite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with harkerite?+

Harkerite commonly co-occurs with Calcite, Gehlenite, Spinel, Fassaite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does harkerite form in?+

Harkerite typically forms in contact metamorphic limestone (skarns). Working float back to the host body is the standard way to chase a fresh occurrence.

What is harkerite used for?+

Harkerite is used in collector.

Find harkerite on the map

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