Natural vs. Lab-Grown Spinel: The “Maltese Cross” Inclusions That Prove Your Spinel Is a Worthless Synthetic Fake.

Spinel can be a treasure or a trap. Natural spinel—especially vivid reds, pinks, and cobalt blues—can be rare and valuable. Lab-grown spinel, by contrast, is mass-produced and inexpensive. One optical clue tips the balance in seconds: the “Maltese cross” seen under crossed polarizers. If you see a clean, centered Maltese cross in a spinel, you are almost certainly looking at a flame-fusion synthetic. Here’s what that means, why it appears, and how to check your stone the right way.

What spinel is—and why so much of it is synthetic

Spinel is a magnesium aluminum oxide (MgAl₂O₄). It crystallizes in the cubic system and is singly refractive. Nature makes it in metamorphic marbles and alluvial deposits. Fine natural crystals are uncommon, and top stones command strong prices.

Lab-grown spinel is easy and cheap to produce by the Verneuil (flame-fusion) method. You melt powdered chemicals and let a “boule” grow as droplets fuse. This creates large, clean crystals with strong internal stresses. That stress is the key to the Maltese cross. Flux and Czochralski methods can also grow spinel, but the jewelry market is dominated by flame-fusion material because it’s fast and low cost.

The Maltese cross: what it is and why it forms

The Maltese cross is not a literal inclusion. It’s an optical strain pattern that appears when you view a flame-fusion spinel between crossed polarizers (for example, in a polariscope or with two polarized filters). You’ll see four dark arms meeting in the center of the stone, creating a symmetric, cross-shaped figure.

Why it happens:

• Cubic, but stressed: Spinel is cubic and ideally isotropic, so it should go dark under crossed polars. Flame-fusion growth and rapid cooling lock in strain that breaks that ideal behavior.
• Radial stress from the boule: The thermal gradient during growth creates a radial, symmetric stress field. Under crossed polars, that stress turns the otherwise dark field into a bright pattern with a dark cross.
• Symmetry matters: Because the stress is evenly distributed, the cross is centered and crisp. Natural spinel may show minor, patchy anomalous double refraction (ADR), but it does not produce a perfect, centered Maltese cross.

In gemology practice, a distinct Maltese cross in spinel is considered diagnostic of flame-fusion origin. That’s why professionals rely on it—because the physics of Verneuil growth creates the pattern, and nature does not.

How to check your stone at home

You can look for the Maltese cross with simple tools. A professional polariscope is best, but you can improvise.

• Option A: Two polarizing filters. Use two cheap linear polarizing films, or two pairs of polarized sunglasses. Hold one fixed and rotate the other until they go dark (crossed). Place the stone between them and look through the stack.
• Option B: Phone screen + polarized sunglasses. A phone screen emits polarized light. Display a white screen. Wear polarized sunglasses. Hold the stone in front of the screen and rotate the phone or glasses until the background looks darkest. Observe the stone.

What to look for:

• Clean, centered cross: Four dark arms meeting in the middle, seen through the table. As you rotate the stone or the top polarizer, the cross stays centered and obvious.
• Not just random strain: Natural stones can show patchy light-and-dark or irregular streaks, especially if strained. That is not a Maltese cross.
• Check in multiple positions: Test through the table and also from the side. Flame-fusion stones often show the cross in several orientations.

If you see a textbook Maltese cross, you can be confident it’s a flame-fusion synthetic spinel.

What the cross proves—and what it doesn’t

It proves flame-fusion synthetic spinel. The symmetric, centered Maltese cross in spinel is a signature of Verneuil growth. That’s why gemologists treat it as a clincher.

It does not appear in natural spinel. Nature can produce strain, but it doesn’t create the even, boule-style stress that yields the Maltese cross.

It won’t identify other synthetics. Flux-grown spinel rarely shows a Maltese cross because the internal stress pattern is different. Those stones need other tests (see below).

Beware of glass. Glass can show strain patterns between crossed polars, but they look different—irregular, not a neat Maltese cross. RI and heft also separate glass from spinel.

Other clues to separate natural and synthetic spinel

A good identification stacks multiple observations. Here are reliable cross-checks and why they work.

• Curved growth lines (curved striae): Flame-fusion growth lays down successive layers that arc as the boule rotates. Under a loupe or microscope with reflected light, you may see faint, curved bands, especially from the side. Natural spinel grows in straight crystal faces, so you see straight or angular zoning if any.
• Gas bubbles: Verneuil stones can trap round bubbles along growth bands. Natural spinel may have negative crystals, but they are angular and aligned with crystal directions, not perfect spheres.
• Flux inclusions (if it’s flux-grown): Wispy “veil” residues, fingerprint-like clouds, or tiny metallic platelets can signal flux synthetics. Natural spinel usually shows crystals, healed feathers, and straight twin lamellae along octahedral planes.
• Refractive index and specific gravity: Spinel’s RI is ~1.718 and SG ~3.58. Glass is lower in SG and often lower RI; doublets or composites can mismatch. A gem shop can measure these quickly.
• UV fluorescence: Chromium-bearing red spinel often fluoresces. Both natural and synthetic can glow, so color alone is not diagnostic; however, in combination with other signs it helps build the case.
• Twinning/parting: Natural spinel commonly shows straight parting planes on {111}. Seen as straight lines or steps under magnification. Synthetics often lack this crisp, natural twinning texture.

Is a synthetic spinel “worthless”?

From a collector’s standpoint, flame-fusion spinel has little market value. It costs pennies to make and exists by the ton. That is why jewelers often find it in older rings as a stand-in for sapphire or aquamarine. If you paid natural-stone prices for it, you overpaid.

But “worthless” is too harsh in everyday terms. Synthetic spinel is durable, bright, and ethical to source. It makes attractive, affordable jewelry when disclosed. Its problem is misrepresentation, not beauty.

Buying checklist: how to avoid paying natural prices for a synthetic

• Ask for origin disclosure in writing. The invoice should say “synthetic spinel” if it’s lab-grown. “Spinel” alone implies natural by trade standards.
• Look through crossed polars. If the seller has a polariscope, ask to view the stone. A Maltese cross ends the debate.
• Inspect under magnification. Search for curved striae and gas bubbles. Natural spinel should show natural-style inclusions and straight growth features.
• Request a lab report for expensive stones. For fine reds or cobalt blues, a report from a recognized lab is prudent. That small cost prevents a large mistake.
• Confirm the return policy. If later testing shows the stone is synthetic, you need a clear path to a refund.

Quick answers to common questions

• Can natural spinel ever show a Maltese cross? No. Natural spinel can show uneven strain, but not the symmetric, centered Maltese cross that flame-fusion stones show.
• Can mounting hide or create the cross? A tight setting can add minor strain, but it won’t generate the clean Maltese cross. You can often see the cross even in mounted stones by viewing through the table.
• Will heating or recutting remove the cross? Not reliably. The stress pattern is inherent to how the boule formed. Some strain may relax, but the diagnostic pattern typically remains.
• Do all synthetics show the cross? No. It’s characteristic of Verneuil spinel. Flux-grown or Czochralski synthetics may not show it and require other tests.

Bottom line

If you see a crisp, centered Maltese cross in your spinel between crossed polarizers, you are looking at a flame-fusion synthetic. That single observation is one of gemology’s most reliable shortcuts. Use it, then confirm with magnification and, for valuable stones, a lab report. You’ll protect your budget—and you’ll know exactly what you’re wearing.