Chrome Pyrope Identification Guide

What Chrome Pyrope Looks Like

Chrome pyrope is the chromium-colored variety of pyrope garnet (Mg3Al2(SiO4)3). It displays a bright, slightly purplish to blood red, often more vivid and "fiery" than ordinary pyrope thanks to chromium. Luster is vitreous to subadamantine, and it is typically transparent to translucent. As an isometric (cubic) mineral it tends to form rounded grains, dodecahedra, or trapezohedra, and is frequently recovered as water-worn pebbles or as rounded grains in kimberlite concentrate.

Step-by-Step Field ID Checklist

1. Note the red color. Look for a lively red to purple-red with strong internal fire under light.
2. Check the shape. Rounded equant grains or 12-sided dodecahedral crystals point to garnet.
3. Test hardness. Mohs 7-7.5 — scratches glass readily.
4. Look for cleavage. Garnet has no cleavage; breakage is conchoidal/uneven.
5. Feel the weight. Pyrope is dense (SG ~3.7-3.8), heavier than quartz of the same size.
6. Check single refraction. Garnet is isometric, so no doubling of back facets (unlike zircon or peridot).

Key Diagnostic Tests

• Hardness: 7-7.5.
• Cleavage: None; conchoidal fracture.
• Streak: White.
• Density: ~3.7-3.8 g/cm3 (chrome pyrope toward the higher end).
• Magnetism: Non-magnetic to very weakly attracted by a strong magnet (iron content).
• Acid: No reaction.
• Optics: Singly refractive; chrome pyrope may show a chromium absorption spectrum and can appear red through a Chelsea filter.

Common Look-Alikes and How to Tell Them Apart

• Almandine / rhodolite garnet: Also garnets and hard to separate by eye; chrome pyrope is usually a purer, brighter red and is denser than almandine in some cases. Lab spectra distinguish chromium coloration.
• Ruby: Corundum is harder (9), denser (~4.0), and doubly refractive; rubies fluoresce red under UV more strongly.
• Spinel (red): Spinel is also isometric and singly refractive; separation needs RI (spinel ~1.72 vs pyrope ~1.74) or density.
• Pyrope vs glass: Glass shows bubbles, lower hardness, and conchoidal swirl marks.
• Arizona/Cape "ruby" (also pyrope): These are trade names for the same chrome pyrope species.

Where It Is Typically Found

Chrome pyrope is a classic kimberlite indicator mineral, used by diamond prospectors because its presence (especially the subcalcic "G10" composition) signals diamond-bearing source rocks. It is found in and around kimberlite pipes and their alluvial trains in South Africa, Russia (Yakutia), the western United States (Arizona's "Anthill garnets" picked up by ants, Colorado, Wyoming), and other cratonic regions. It also occurs in chromium-rich peridotite and serpentinite.

Field Tips and Common Mistakes

When you find small rounded red grains in dark soil near a known kimberlite, do not assume "ruby" — chrome pyrope is far more common in that setting and is what indicator-mineral sampling targets. The quickest separator from ruby and red spinel is double refraction: examine the stone with a loupe through a polariscope or look for doubled facet edges, which pyrope (singly refractive) lacks but ruby shows. A strong neodymium magnet can give a faint tug to iron-bearing garnet, helping you sort garnet from glass and from corundum. Because chrome pyrope can look almost identical to almandine and rhodolite, treat exact garnet species as a lab question; in the field, classify by color (bright purplish-red), hardness, single refraction, and density. Beware of glass imitations in tumbled-stone lots: they show swirl lines, gas bubbles, and rounded "conchoidal" mold marks, and they are softer than 7.