Kimberlite Identification Guide

What Kimberlite Looks Like

Kimberlite is the famous diamond-host volcanic rock—a potassic, volatile-rich ultramafic rock that erupts from the deep mantle through narrow "pipes." Fresh kimberlite ("blue ground") is dark bluish-gray to greenish-gray; weathered near-surface kimberlite ("yellow ground") is yellowish-brown and friable. Texturally it is inequigranular and brecciated, with rounded olivine crystals (often altered to greenish serpentine), bronzy phlogopite mica, and scattered mantle xenocrysts—including red-purple pyrope garnet, green chrome-diopside, ilmenite, and chromite—set in a fine carbonate-serpentine matrix. It commonly carries xenoliths of mantle peridotite and crustal rock.

Quick visual cues

• Dark, mottled, brecciated ultramafic rock (or yellow, crumbly when weathered)
• Rounded olivine grains, often serpentinized green
• Bronzy phlogopite mica plates
• Indicator minerals: red pyrope garnet, emerald-green chrome diopside, black ilmenite

Step-by-Step Field ID Checklist

1. Look for a pipe/diatreme setting with brecciated, xenolith-rich rock.
2. Identify olivine (rounded, may be altered to serpentine).
3. Find phlogopite bronze mica flakes.
4. Hunt indicator minerals: bright red-purple pyrope garnet and grass-green chrome diopside are classic diamond-pathfinders.
5. Note the matrix: fine-grained serpentine + carbonate; often soft and altered.
6. Test for carbonate: the groundmass commonly fizzes with dilute acid.

Key Diagnostic Tests

• Acid: carbonate-bearing matrix often effervesces in dilute HCl (unusual for an ultramafic rock—useful clue).
• Hardness: variable; altered matrix is soft (serpentine ~3-4), olivine ~6.5-7, garnet 7+.
• Density: moderately high (~2.7-3.3) but altered kimberlite can be light and crumbly.
• Magnetism: often weakly magnetic from magnetite/ilmenite.
• Indicator-mineral panning: concentrate heavy minerals and look for pyrope, chrome diopside, picroilmenite, and chromite.

Common Look-Alikes and How to Tell Them Apart

• Lamproite: the other diamond host; lamproite has leucite/sanidine-type chemistry and lacks a carbonate-dominant matrix—needs lab work, and lamproites occur in different settings (e.g., Argyle).
• Serpentinite: green and soft like altered kimberlite but lacks the brecciation, mantle xenocrysts, and indicator-mineral suite.
• Basalt/peridotite: lack the pipe form, phlogopite, carbonate matrix, and diamond-indicator minerals.
• Lamprophyre: mica-rich but dike-forming and crustal, without mantle garnet/diopside xenocrysts.

Where It Is Found

Kimberlite occurs as pipes and dikes in stable cratons: classic localities include Kimberley, South Africa (the namesake), plus Russia (Yakutia/Mir), Canada (Ekati, Diavik), and the USA (Crater of Diamonds, Arkansas). It is the world's principal primary source of diamonds.