Cape Ruby Identification Guide
How to recognize Cape Ruby, the deep red pyrope garnet from South Africa, and separate it from true ruby and other red garnets.
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What Cape Ruby Looks Like
"Cape Ruby" is a trade name for pyrope garnet, not corundum ruby. It is a deep blood-red to slightly brownish-red or purplish-red stone, often quite dark in larger sizes. Cape Ruby is closely tied to the South African diamond fields, where it occurs as small, water-worn grains and is sometimes called a diamond "indicator mineral."
- Color: dark red, blood-red, sometimes with a brown or violet undertone.
- Luster: vitreous (glassy).
- Transparency: transparent to translucent; deeper stones can look nearly opaque.
- Crystal habit: isometric; rounded dodecahedral grains, or anhedral rolled pebbles.
Step-by-Step Field-ID Checklist
- Confirm it is a garnet, not corundum. Check for the absence of cleavage and a single (isotropic) optical character.
- Test hardness: garnet is about 7–7.5, softer than ruby (9). It will scratch quartz but not corundum.
- Look at the color tone: Cape Ruby reads as a pure-to-brownish red, lacking the strong pink/fuchsia overtone of fine ruby.
- Check size and form: genuine Cape pyrope is usually small (under a carat or two) and may be water-rolled.
- Streak: white.
Key Diagnostic Tests
- Mohs hardness: 7–7.5.
- Streak: white.
- Cleavage/fracture: no cleavage; conchoidal to uneven fracture.
- Magnetism: pyrope is weakly attracted to a strong neodymium magnet because of its iron/magnesium content — a useful garnet test that ruby fails.
- Optical: singly refractive (isotropic); stays dark between crossed polarizers. Ruby is doubly refractive.
- Density: roughly 3.6–3.8 g/cm³.
Common Look-Alikes
- True ruby (corundum): harder (9), doubly refractive, shows pleochroism, and never responds to a magnet. The single biggest tell.
- Almandine and rhodolite garnet: also red but typically denser; rhodolite shows a distinct purplish-raspberry hue, almandine is darker brownish-red. Spectroscopy and density help separate them, but all are garnets so the trade overlap is real.
- Red spinel: also isotropic and hard (8); responds far more weakly to a magnet and has different inclusions.
- Red glass/synthetics: gas bubbles, swirl marks, warm to the touch.
Where Cape Ruby Is Found
Classic sources are the Kimberley diamond region of South Africa, where pyrope occurs in and around kimberlite pipes. Pyrope of similar quality also comes from Arizona ("Arizona ruby"), Bohemia, and Tanzania.
Frequently asked questions
Is Cape Ruby a real ruby?
No. "Cape Ruby" is a marketing name for red pyrope garnet from South Africa. It is not corundum, so it is softer (Mohs 7–7.5 vs 9) and chemically unrelated to true ruby.
How can you tell Cape Ruby from a real ruby?
Real ruby is harder (scratches the garnet), is doubly refractive with visible pleochroism, and never reacts to a magnet. Cape Ruby pyrope is singly refractive, softer, and shows a slight pull toward a strong neodymium magnet.
What does Cape Ruby look like?
A deep blood-red to brownish- or purplish-red transparent stone with a glassy luster, usually small and sometimes water-worn, mined alongside South African diamonds.
Is Cape Ruby valuable?
It is an affordable garnet, far cheaper than corundum ruby. Clean, well-colored stones have modest gem value, but it is prized historically as a diamond indicator mineral.