Knorringite Identification Guide
Identify Knorringite, a rare chromium-rich pyrope garnet and diamond indicator, by its green color and kimberlite setting.
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What Knorringite Looks Like
Knorringite is a rare chromium-rich garnet, the Cr end member of the pyrope series (ideally Mg3Cr2(SiO4)3). Pure knorringite is essentially unknown; in nature it occurs as a component of green, Cr-bearing pyrope garnets. These garnets are typically emerald-green to grass-green or yellow-green, transparent to translucent, with a vitreous luster and the usual garnet isometric form (rounded grains or dodecahedra). Knorringite-rich pyrope is a prized diamond indicator mineral.
Quick visual cues
- Bright green to yellow-green garnet grains
- Glassy luster, no cleavage
- Small, often rounded mantle-derived grains
- Recovered from kimberlite and stream concentrates
Step-by-Step Field ID Checklist
- Confirm garnet form: isometric, no cleavage, conchoidal fracture.
- Note the green color: Cr gives a vivid green—distinct from red pyrope/almandine.
- Check the source: knorringitic pyrope comes from kimberlite/mantle xenoliths and heavy-mineral concentrates.
- Test hardness: hard, Mohs ~7.5, scratches glass and quartz.
- Heft: high density (~3.7-3.8) typical of garnet.
- Look at grain size: usually small xenocrysts rather than large gem crystals.
Key Diagnostic Tests
- Mohs hardness: ~7-7.5.
- Density: ~3.7-3.8 g/cm^3.
- Cleavage/fracture: none/conchoidal.
- Streak: white.
- Acid: no reaction.
- Definitive ID: microprobe—high Cr2O3 (and the G10/G9 garnet classification) identifies diamond-associated knorringitic pyrope.
Common Look-Alikes and How to Tell Them Apart
- Uvarovite: the calcium-chromium garnet—also green, but Ca-rich and usually as tiny bright crystals on chromite; knorringite is Mg-rich pyrope from the mantle (lab chemistry separates them).
- Chrome diopside: another green kimberlite indicator, but it is a pyroxene with two cleavages (~87 degrees), whereas knorringite garnet has none.
- Peridot (olivine): green and gem-like but has lower hardness (~6.5-7), no cleavage, and a distinctive olive hue; optics/SG differ.
- Tsavorite (green grossular): gem green garnet but Ca-Al with no mantle/Cr-pyrope context; locality and chemistry differ.
Where It Is Found
Knorringite-bearing pyrope occurs in kimberlites and mantle peridotite xenoliths (type material from the Kao kimberlite, Lesotho) and is recovered in diamond-exploration heavy-mineral surveys worldwide. It is a research/indicator mineral, not a faceted gem.
Frequently asked questions
How do you identify Knorringite?
It appears as bright green-to-yellow-green isometric garnet grains with no cleavage, a hardness around 7.5, high density, and a white streak, recovered from kimberlite or heavy-mineral concentrates; microprobe analysis confirms its high chromium content.
Why is Knorringite important for diamond exploration?
Chromium-rich (knorringitic) pyrope garnet forms in the same deep mantle conditions as diamonds, so geologists use it as an indicator mineral when prospecting for diamond-bearing kimberlites.
Knorringite vs chrome diopside—how do they differ?
Both are green kimberlite indicator minerals, but knorringite is a garnet with no cleavage and a hardness near 7.5, while chrome diopside is a pyroxene with two cleavage directions and lower hardness.
What color is Knorringite?
Knorringite-rich pyrope is emerald-green to grass-green or yellow-green, colored by chromium, contrasting with the red of ordinary pyrope and almandine garnet.