Home Gemstone Testing: 5 Simple Tests You Can Do to Spot a Lab-Grown Stone, Don’t Get Fooled by a “Too Perfect” Gem.

Lab-grown gems are real gemstones made in a factory, not fakes. They share the same chemistry and beauty as natural stones. That’s why they can fool you—especially when they look flawless. At home, you can’t prove origin with certainty, but you can spot red flags that point to a lab-grown or an outright imitation. The five tests below are simple, low-cost, and safe for your stones. Use them together to avoid being fooled by a “too perfect” gem.

Before You Start: Why “Too Perfect” Is a Clue

Nature is messy. Most natural gems show some mix of inclusions, slight color zoning, small internal stress, or uneven fluorescence. Lab-grown stones form under controlled conditions, so they often look uniform: even color, minimal inclusions, and crisp, consistent structure. That “perfection” can be a hint, especially when the size and price seem too good to be true.

Important: a clean gem isn’t proof of lab origin, and an included gem isn’t proof of natural origin. You’re looking for patterns—multiple signs that agree.

Tools You’ll Need (Inexpensive, No Damage)

• 10× jeweler’s loupe (triplet preferred) and a small LED penlight
• 365 nm UV flashlight (longwave), plus a dark room and white paper
• Strong neodymium magnet (small cylinder or cube)
• Kitchen scale (0.1 g precision) and a cup of water with thread or hair
• Two polarized lenses (two pairs of polarized sunglasses work)

Test 1 — Loupe and Light: Inclusions and Growth Features

Why it works: Growth environment leaves fingerprints inside a gem. Natural crystals show mineral inclusions, fingerprints, and irregular zoning. Lab-grown often shows curved growth lines, metallic specks, or needle-like patterns that differ from natural.

How to do it:

• Clean the stone. Hold it with tweezers or in a ring clamp.
• Use the loupe at 10×. Backlight with a penlight and view from several angles.
• Scan the whole stone: center, edges, just under the surface, and along the girdle.

What to look for:

• Diamonds:
  • Natural: Small crystals, feathers, graining; inclusions often irregular.
  • Lab-grown (HPHT): Tiny dark metallic specks or reflective platelets (flux). Sometimes blocky growth zones.
  • Lab-grown (CVD): Layered “stacked” growth, dark pinpoint clouds, or parallel striations.
• Ruby/Sapphire (corundum):
  • Natural: Straight growth zoning, rutile “silk” needles in intersecting patterns, uneven color patches.
  • Flame-fusion synthetic: Curved growth lines (curved striae) and “swirly” color bands. This curvature is a strong lab clue.
• Emerald:
  • Natural: Three-phase inclusions (tiny crystal, liquid, and bubble together), jagged “jardin.”
  • Hydrothermal synthetic: Chevron or “herringbone” growth, nail-head spicules, and very neat, parallel zoning.
• “Diamond” alternatives:
  • Moissanite: Under magnification, facet edges may look doubled; tiny needle clouds.
  • Cubic zirconia or glass: Gas bubbles (round and uniform), flow lines, “orange-peel” surface texture near facets.

Why these clues matter: Curved striae do not happen in natural corundum. Metallic specks in a diamond point to HPHT growth. Gas bubbles point to glass. These features form only under certain man-made processes.

Caution: Some lab gems are very clean; some natural gems are very included. Use this as one piece of the puzzle.

Test 2 — UV Flashlight: Fluorescence and Afterglow

Why it works: Trace elements and crystal defects respond to UV. Labs can produce more uniform, stronger responses. Naturals often show patchy or zoned fluorescence that matches their growth.

How to do it:

• In a dark room, place the gem on white paper.
• Shine a 365 nm UV light from above and the side.
• Note color, intensity, and whether it’s even or patchy. Switch off the light and watch for phosphorescence (afterglow).

Typical patterns:

• Diamonds: Natural stones vary from inert to weak/strong blue. Some rare naturals phosphoresce. HPHT-grown can show strong yellow-green or blue and sometimes a brief afterglow. CVD-grown may show orange-to-red or uneven blue; some phosphoresce weakly. Uniform, unusual colors or obvious afterglow raise suspicion.
• Ruby/Sapphire: Flame-fusion rubies often glow a strong, even, “stop-sign” red. Naturals tend to be weaker and patchy due to zoning or iron content. Sapphires may show chalky blue-white in synthetics; many naturals are weak or inert.
• Emerald: Many natural emeralds are inert (iron quenches fluorescence). Some synthetics show brighter red. If an emerald glows bright, even red, consider hydrothermal origin—though some low-iron naturals can glow too.

Caution: Fluorescence alone is not proof. Treat UV as a supporting clue, especially when the glow is strong and uniformly distributed.

Test 3 — Magnet Check: Flux Residue in HPHT Diamonds

Why it works: HPHT-grown diamonds often use metallic flux (iron, nickel, cobalt). Tiny trapped particles can make the stone weakly magnetic. Natural and CVD diamonds are usually not magnetic.

How to do it:

• Hang a small neodymium magnet from thread so it swings freely.
• Bring the stone close to the magnet and slowly pass it by. Keep metal settings away if possible.
• Watch for any deflection or attraction.

What to expect:

• Attraction: Suggests HPHT diamond or a gem with metallic inclusions. This is a strong lab-grown clue in diamonds.
• No attraction: Could be natural diamond, CVD diamond, moissanite, CZ, sapphire, etc.

Caution: Test loose stones if you can; metal mountings invalidate the test. Very tiny diamonds may be too light to show a response even if HPHT-grown.

Test 4 — Density at the Kitchen Sink: Simple Specific Gravity

Why it works: Different materials have different densities. While lab-grown and natural of the same species share density, this test easily spots impostors (for example, CZ masquerading as diamond).

How to do it:

• Weigh the clean stone dry on a kitchen scale. Note the mass in grams (Wdry).
• Hang the stone from a hair or thin thread so it’s submerged but not touching the cup. Weigh again with the stone fully underwater (Wwet).
• Compute SG ≈ Wdry ÷ (Wdry − Wwet).

Reference values (approximate): Diamond ~3.5; Moissanite ~3.2; CZ 5.6–6.0; Sapphire/Ruby ~4.0; Emerald 2.7–2.8; Spinel ~3.6; Quartz 2.65; Glass 2.4–2.6.

How to use it: If a “diamond” reads near 5.8, it’s CZ. If a “sapphire” reads 2.6, it’s likely glass. If a “diamond” reads ~3.5, it could be natural or lab-grown diamond (this test won’t separate those two).

Caution: Accuracy drops with very small stones or heavy mounts. Use this as a screening tool to eliminate mismatches.

Test 5 — Optics You Can See: Doubling, Read-Through, and Curved Striae

Why it works: Some gems split light into two rays (double refraction). Others don’t. Certain man-made processes also leave curved growth structures visible with simple light.

How to do it:

• Facet doubling (moissanite vs diamond): Look through the table with a 10× loupe. Slowly rock the stone. If you see doubled facet edges (two outlines) inside, it’s likely moissanite (doubly refractive). Diamond is singly refractive, so facet edges remain single.
• Read-through “dot” check on cabochons: Place a small dot on paper. Put a smooth cabochon over it. If you can read the dot clearly, think glass or quartz; if it blurs or disappears, it may be a higher-RI gem like sapphire. (This is rough guidance, not definitive.)
• Curved striae in corundum: Use a penlight in a dim room. Tilt the ruby/sapphire. Look for curved growth lines or color bands. Curved = classic flame-fusion synthetic; straight = natural or hydrothermal/flux-grown.

Caution: Faceting can mask doubling in weakly birefringent stones; rotate and look along multiple directions. The read-through trick is reliable only for smooth cabochons or unfaceted rough.

Bonus: Cross-Polarized Sunglasses for a Quick Optics Check

Why it works: Two polarized lenses at 90° go dark. A doubly refractive gem between them leaks light as you rotate it, creating bright/dark flashes. Singly refractive gems stay mostly dark.

How to do it:

• Hold one pair of polarized sunglasses in front of you. Rotate a second pair 90° so the view goes dark.
• Place the stone between the lenses and slowly rotate the stone.

Read the result:

• Mostly dark (with minor flicker): Diamond, spinel, glass.
• Bright/dark flashing as you rotate: Moissanite, sapphire, ruby, emerald.

Lab-grown clues: Some CVD diamonds show banded or uneven stress patterns; flame-fusion corundum can show smooth, sweeping patterns rather than the patchy mosaic often seen in naturals. Treat this as supportive, not decisive.

Putting Results Together: A Simple Way to Decide

• Looks flawless + perfectly even color + strong, uniform UV glow: Suspicious for lab-grown.
• Diamond with tiny metallic specks under loupe or weak magnet attraction: HPHT-grown likely.
• “Diamond” with facet doubling or SG near 5.8: not diamond (moissanite or CZ).
• Ruby/Sapphire with curved striae: flame-fusion synthetic.
• Emerald with chevron growth or bright, even red UV: hydrothermal synthetic possible.

One clue rarely settles it. Two or three that agree give you a strong indication.

Limits of Home Tests and When to See a Gem Lab

Home tests can separate obvious synthetics and simulants from naturals, but they can’t always tell natural vs lab-grown when chemistry is identical (like natural vs lab diamond or sapphire). Professional labs use spectroscopy, photoluminescence, advanced microscopy, and reference databases. If the stone is valuable or the results conflict, get a lab report.

Also check for laser inscriptions on diamonds (often on the girdle) that identify lab growth and the maker’s report number.

Quick Reference: Red Flags of a “Too Perfect” Stone

• High clarity and large size at a bargain price (especially in diamond, ruby, sapphire, emerald)
• Uniform, saturated color from edge to edge with no zoning
• Curved growth lines in ruby/sapphire
• Strong, even fluorescence that doesn’t match known natural patterns
• Metallic specks or weak magnetism in “diamond”
• Facet doubling in a “diamond” (points to moissanite)
• Density mismatch (e.g., “diamond” reading like CZ)

Trust patterns, not single clues. When in doubt, pause the purchase and get a professional opinion. These simple tests won’t turn you into a gemologist overnight, but they will keep you from being fooled by a gem that looks a little too perfect.