Ishikawaite is a rare, radioactive niobium-tantalum mineral found primarily in granitic pegmatites. It usually presents as black to brownish-black prismatic crystals or irregular masses, often requiring a Geiger counter for positive identification in the field.

Hardness

5-6

Mohs

Luster

Waxy, resinous to vitreous

Streak

Dark brown to black

Transparency

Opaque

mineralrareSamarskite Group family

Ishikawaite

Hardness: 5-6
Luster: Waxy, resinous to vitreous
Streak: Dark brown to black
Transparency: Opaque

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Is this ishikawaite?

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 ishikawaite with a known reference. Ishikawaite sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: dark brown to black.
3
Read the luster
Hold the specimen under a strong light. Luster: waxy, resinous to vitreous.
4
Match the color range
Compare against the expected color range: black.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: metamict.

Often confused with

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

mineral

Columbite

Mohs 6

How to tell apart: Streak differs, so Ishikawaite leaves dark brown to black, Columbite leaves blackish brown; luster reads waxy, resinous to vitreous on Ishikawaite and submetallic on Columbite.

Compare →

Often found alongside ishikawaite

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

Columbite-(Fe)
mineral · Mohs 6

All properties

Chemical formula: (U,Fe,Y)NbO₄
Mohs hardness: 5-6
Density: 6.2-6.4 g/cm³
Colors: Black
Streak: Dark brown to black
Luster: Waxy, resinous to vitreous
Transparency: Opaque
Crystal system: Orthorhombic
Crystal habit: Metamict
Cleavage: None
Rarity: Rare
Uses: Collector, Research
Host rock: In pegmatite, and alluvium (Ishikawa district, Japan); as micro-inclusions in vitrophyre clasts in highly differentiated, rare-element rich, pyroclastic rhyolites (Honeycomb Hills, USA)

Where rockhounds find ishikawaite

Classic worldwide localities

Ishikawa district, Fukushima Prefecture, Japan
Reported from other localities in Japan, but confirmation is impossible
Kunar, Afghanistan
Honeycomb Hills, Juab Co., Utah, USA, for example, seems very similar but is regarded as uranoan samarskite

Field-hunting tip

Where it forms: In pegmatite, and alluvium (Ishikawa district, Japan); as micro-inclusions in vitrophyre clasts in highly differentiated, rare-element rich, pyroclastic rhyolites (Honeycomb Hills, USA). If you start seeing samarskite, ferrocolumbite in float, you are in the right ground. Typical habit: metamict. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify ishikawaite?+

Mohs hardness is 5-6. Luster: waxy, resinous to vitreous. The streak is dark brown to black. Common colors include black.

Where is ishikawaite found?+

Notable localities include Ishikawa district, Fukushima Prefecture, Japan; Reported from other localities in Japan, but confirmation is impossible; Kunar, Afghanistan; Honeycomb Hills, Juab Co., Utah, USA, for example, seems very similar but is regarded as uranoan samarskite.

Is ishikawaite safe to handle?+

Ishikawaite contains uranium or thorium and calls for real care. This mineral is radioactive and contains uranium; it should be stored in a lead-lined or shielded container and handled with proper precautions, including wearing gloves and minimizing inhalation of dust.

What rocks look like ishikawaite?+

Ishikawaite is most often confused with Columbite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with ishikawaite?+

Ishikawaite commonly co-occurs with Samarskite, Ferrocolumbite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does ishikawaite form in?+

Where it forms: In pegmatite, and alluvium (Ishikawa district, Japan); as micro-inclusions in vitrophyre clasts in highly differentiated, rare-element rich, pyroclastic rhyolites (Honeycomb Hills, USA). Working float back to the host body is the standard way to chase a fresh occurrence.

What is ishikawaite used for?+

Ishikawaite is used in collector, research.

Find ishikawaite on the map

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