Spinel is recognised as one of the world’s most remarkable gemstones, celebrated for its intense colour and sparkling brilliance. Historically, it was often mistaken for ruby or sapphire; for example, the famous “Black Prince’s Ruby” in the British Crown Jewels is actually a red spinel.
Unlike corundum, spinel is singly refractive and belongs to the cubic crystal system, just like diamonds. This crystal structure provides spinel with a sharper and cleaner glitter compared to many other coloured stones. With a Mohs hardness of 8, spinel is durable enough for everyday wear, yet remains undervalued given its exceptional beauty and rarity.
When selecting a spinel gemstone, consider these five key factors to ensure you’re identifying a high-quality stone:
1. Evaluate the Cut – Watch for “Windowing”
Spinel commonly forms in naturally flat, twinned crystals. In some cases, cutters may prioritise retaining weight over achieving beauty, which can result in stones that are too shallow. This leads to “windowing” a transparent area in the middle where light passes directly through, rather than being reflected back.
How to check: If you can read through the centre of the stone, it has a window.
Why it matters: Windowing diminishes a gemstone’s brilliance and overall visual appeal.
2. Assess Colour Saturation in Natural Light
Spinel is available in a wide spectrum of colours: vivid reds, hot pinks, cobalt blues, lavender, and more. The lighting conditions can dramatically impact how a stone appears, making it crucial to evaluate colour in natural daylight.
Common issue: Spinel stones can often appear too dark or nearly black under normal lighting conditions.
Recommendation: Always examine spinel in natural daylight. If a stone appears dark in daylight, it will likely look even darker once set in metal.
3. Inspect for Facet Chipping and Surface Wear
While spinel is durable, its facet edges and table remain vulnerable to chipping during cutting or handling.
What to look for:
- Tiny white nicks along facet edges
- “Fuzzy” or uneven facet lines
- Surface reaching inclusions or fractures
Spinel is typically eye-clean, so visible damage or clarity issues should be treated seriously.
4. Understand the Single-Refraction Sparkle
One of spinel’s greatest assets is its optical structure. Being singly refractive, spinel produces a crisp, sharp sparkle, reminiscent of diamonds and garnets.
What this means: The sparkle is clear and well-defined, setting it apart from stones with multiple refractions.
Why it’s valuable: This unique optical quality enhances spinel’s desirability and visual impact.
5. Check for Treatments and Synthetics
Though most spinels available are natural and untreated, certain enhancements and synthetics are present in the market.
Treatments to be aware of:
- Low-temperature heating (noted in some Tanzanian stones)
- Cobalt diffusion, used to mimic rare natural cobalt-blue spinel
Synthetics: Synthetic spinel has been produced for decades. These stones often display:
- No inclusions
- A strong red UV reaction
- Colour that appears “too perfect” for the price
If a spinel appears flawless and is unusually inexpensive, it may be synthetic.
