Idaho Star Garnet Identification Guide
How to identify Idaho star garnet, the asteriated almandine that shows a four- or six-rayed star, plus its tests and look-alikes.
Read the full Idaho Star Garnet encyclopedia entry →
What Idaho Star Garnet Looks Like
Idaho star garnet is almandine garnet (iron-aluminum garnet) that displays asterism — a star of light across its surface caused by oriented rutile/ilmenite needle inclusions. It is the Idaho state gem:
- Color: deep red-brown, purplish-red, to nearly black; usually dark and opaque
- Luster: vitreous to slightly resinous
- Transparency: translucent to opaque
- Habit: rounded waterworn grains and dodecahedral crystals; cut as cabochons to reveal the star
The hallmark is a four-rayed (sometimes six-rayed) star floating on a polished dome under a single light.
Step-by-Step Field ID Checklist
- Confirm garnet color and form. A dark red-purple, equant, glassy pebble in mica-rich gravels is the first clue.
- Look for the star. Wet the stone or shine a single light on a rounded face; oriented inclusions produce a 4- or 6-ray star.
- Test hardness. Mohs 7–7.5; scratches glass and quartz easily.
- Check for no cleavage and conchoidal fracture.
- Confirm the setting. Idaho stones come from mica schist gravels worked by panning/digging.
Key Diagnostic Tests
- Asterism: the diagnostic feature; four-ray stars are characteristic (Idaho is one of the few places producing them), with rarer six-ray stars.
- Hardness: 7–7.5.
- Cleavage: none; fracture conchoidal.
- Streak: white.
- Density: high, ~3.9–4.2 g/cm3, so it feels heavy.
- Magnetism: iron-rich almandine is weakly attracted to a strong neodymium magnet.
- No acid reaction.
Common Look-Alikes
- Star sapphire/ruby: harder (9), and corundum stars are usually six-ray; garnet is softer and dark red-purple.
- Ordinary almandine garnet: same chemistry but lacks the oriented inclusions, so it shows no star.
- Star diopside: softer (5.5–6.5), typically black-green, four-ray star but much lighter in thin areas.
- Spinel/black tourmaline pebbles: lack a star and differ in density and magnetism.
- Rhodolite garnet: more transparent purplish-red and rarely shows asterism.
Where It Is Found
The premier source is the Emerald Creek / St. Maries area of northern Idaho (Latah and Benewah counties), where star garnets are dug and screened from streambed gravels derived from mica schist. The only other significant four-ray star garnet source is India.
Frequently asked questions
How can you tell if it's real Idaho star garnet?
Wet or polish a rounded face and shine a single light on it: a real star garnet shows a four- or six-rayed star of light. It is a hard (7–7.5), heavy, dark red-purple garnet with no cleavage, conchoidal fracture, and weak attraction to a strong magnet.
Why do Idaho star garnets show a star?
They contain microscopic needle-like rutile or ilmenite inclusions arranged in crystallographic directions. When light reflects off these aligned needles on a curved polished surface, it forms a star (asterism).
What does Idaho star garnet look like?
It looks like a dark red-brown to purplish, glassy, rounded pebble or crystal; when polished into a dome it reveals a silvery four- or six-rayed star under direct light.
Where can you find star garnets in Idaho?
The main public site is the Emerald Creek Garnet Area near Fernwood and St. Maries in northern Idaho, where you screen them from stream gravels. India is the only other notable source of four-ray star garnets.
Idaho Star Garnet identified by the community
Recent Idaho Star Garnet specimens identified with Rock Identifier.
