Comparison

Few mix-ups in the history of gemology are as famous as the confusion between spinel and ruby. For centuries, spinels were classified as rubies — and some of the most celebrated "rubies" in royal treasuries around the world are, in fact, spinels. The most famous example is the 170-carat "Black Prince's Ruby" set in the British Imperial State Crown — which is a red spinel. The "Timur Ruby" in the same crown is also a spinel. Both were treasured for hundreds of years under a false identity.

Sri Lanka produces both gems — and the confusion continues today in markets, online shops, and even among relatively experienced buyers. Understanding how to separate these two gemstones is one of the most practically valuable skills a gem buyer can develop.

The Critical Difference: Crystal System

The most fundamental difference between ruby and spinel is their crystal system. Ruby is corundum, which crystallises in the trigonal system and is therefore doubly refractive — light splits into two rays as it passes through the stone. Spinel crystallises in the cubic (isometric) system and is singly refractive — light travels through it as a single ray.

This difference is detectable with basic gemological tools. Under a polariscope or even just by looking through a loupe at the back facets of the stone, doubly refractive ruby shows facet-edge doubling — the back facet edges appear as double lines. Singly refractive spinel shows no doubling. A dichroscope confirms this: ruby shows two distinct colours (pleochroism); spinel shows the same colour from all directions.

Key Identification Properties

Refractive Index is the most decisive single measurement. Ruby (corundum) has an RI of 1.762–1.770. Spinel has an RI of approximately 1.718 — clearly lower. A single refractometer reading separates them conclusively.

Specific Gravity also differs usefully. Ruby has SG 4.00; red spinel has SG approximately 3.60. A spinel of the same size as a ruby will feel noticeably lighter. This is detectable by hydrostatic weighing or, for experienced handlers, by the feel of the stone.

UV Fluorescence provides a useful clue. Most rubies — particularly Burmese rubies — show strong red fluorescence under longwave UV light, because chromium both colours the stone and causes it to luminesce. Red spinel also contains chromium and can show red fluorescence, but typically weaker and less consistent than ruby. This test is supplementary, not definitive.

Inclusions are highly characteristic under magnification. Ruby typically contains fine rutile silk needles, twinning planes, and sometimes negative crystal inclusions specific to corundum. Spinel tends to show octahedral crystal inclusions and "fingerprint" inclusions of a different character. An experienced gemologist can often distinguish the two from inclusions alone under a microscope.

Ceylon Spinel — A Gem Worth Knowing

Red spinel from Sri Lanka has historically been undervalued simply because it was not ruby — which is unfair. Fine red and hot-pink spinels from Sri Lanka can be extraordinarily beautiful stones with vivid saturation, excellent transparency, and a brightness that rivals ruby. The cubic crystal structure that makes spinel singly refractive also gives it a clean, pure colour without the colour zoning sometimes seen in ruby.

Sri Lanka also produces spinel in a remarkable range of colours: red, hot pink, lavender, blue, violet, and the very rare blue-grey called "cobalt spinel". Cobalt-coloured spinel from Sri Lanka is one of the rarest and most valuable coloured gemstones in the world — even rarer than fine padparadscha — coloured by cobalt rather than the iron/chromium that creates most other spinel colours.

Price Comparison

As a rough guide for natural, untreated stones with laboratory certification:

• Fine Ceylon ruby (2 ct, GRS cert): USD 3,000–15,000 per carat
• Fine red Ceylon spinel (2 ct, GRS cert): USD 800–4,000 per carat
• Fine hot-pink Ceylon spinel (2 ct): USD 500–2,500 per carat
• Cobalt-blue Ceylon spinel (1 ct, GRS cert): USD 5,000–30,000+ per carat

The price gap between ruby and red spinel has narrowed significantly in recent years as collectors have rediscovered spinel's merits. Cobalt spinel has experienced dramatic price appreciation as its rarity has become more widely understood.

The Test Protocol in Summary

If you have a red or pink stone and need to determine whether it is ruby or spinel, follow this sequence: first check RI (ruby 1.762–1.770; spinel 1.718); then check for facet-edge doubling under magnification (ruby shows doubling; spinel does not); then check UV fluorescence (ruby typically stronger); finally examine inclusions under a microscope. If all four indicators point the same way, you have a reliable identification. For any significant purchase, obtain a laboratory report — the GRS or GIA can distinguish them definitively.

Spinel is not a poor man's ruby. It is a magnificent gem in its own right — one that deserves recognition on its own merits rather than simply as a substitute for something else.

Disclaimer: Price ranges are approximate market references. Always obtain independent professional advice before purchasing.