Hardystonite is highly sought after by fluorescent mineral collectors due to its characteristic deep violet-blue glow under shortwave ultraviolet light. It occurs almost exclusively in the famous zinc-manganese orebodies of Franklin and Sterling Hill, New Jersey, typically as massive or granular grains within marble.

Hardness

3-4

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineralrareMelilite Group family

Hardystonite

Hardness: 3-4
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this hardystonite?

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 hardystonite with a known reference. Hardystonite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous.
4
Match the color range
Compare against the expected color range: white, light brown, pink.
5
Look at form & habit
Crystal system: tetragonal. Typical habit: massive.

Often confused with

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

mineral

Willemite

Mohs 5.5

How to tell apart: Willemite is the harder of the two (Mohs 5.5 vs. 3-4).

Often found alongside hardystonite

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

All properties

Chemical formula: Ca₂ZnSi₂O₇
Mohs hardness: 3-4
Density: 3.396-3.443 g/cm³
Colors: White Light brown Pink
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Tetragonal
Crystal habit: Massive
Cleavage: {001}, good; {100} and {110}, poor
Fluorescence: Dark purple under SW UV; intense violet-blue under LW UV
Rarity: Rare
Uses: Collector
Host rock: In granular ore in a metamorphosed stratiform zinc deposit

Where rockhounds find hardystonite

Classic worldwide localities

Franklin, Sussex Co., New Jersey, USA

Field-hunting tip

Where it forms: In granular ore in a metamorphosed stratiform zinc deposit. If you start seeing vesuvianite, apatite, franklinite in float, you are in the right ground. Typical habit: massive. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify hardystonite?+

Mohs hardness is 3-4. Luster: vitreous. The streak is white. Common colors include white, light brown, pink.

Where is hardystonite found?+

Notable localities include Franklin, Sussex Co., New Jersey, USA.

What rocks look like hardystonite?+

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

What minerals are found with hardystonite?+

Hardystonite commonly co-occurs with Vesuvianite, Apatite, Franklinite, Willemite, Rhodonite, Calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does hardystonite form in?+

Where it forms: In granular ore in a metamorphosed stratiform zinc deposit. Working float back to the host body is the standard way to chase a fresh occurrence.

What is hardystonite used for?+

Hardystonite is used in collector.

Find hardystonite on the map

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