"Crystal" is a word that has been stretched to cover almost anything transparent. Lead glass is sold as "crystal" glassware, coloured glass is sold as "crystal" beads, and moulded glass spheres are sold as "crystal balls". Meanwhile the mineral that the word originally described — quartz, silicon dioxide grown as a crystal in the earth — is what buyers of sphatik malas, healing stones and clear quartz points believe they are getting.
The two materials can look identical to the eye. They are not identical to a laboratory, or to anyone with a magnifier and ten minutes, because glass and quartz differ in almost every physical property that can be measured.
The fundamental difference
Quartz is crystalline: its atoms are arranged in a repeating, ordered lattice. Glass is amorphous: its atoms are frozen in the disorder of a liquid. That single fact drives every test below. Crystalline quartz is doubly refractive, has a fixed refractive index and specific gravity, is harder, conducts heat better and shows growth features. Glass is singly refractive, varies in RI and SG with its recipe, is softer, feels warmer, and shows manufacturing features.
What you can check yourself
Bubbles. Look through the piece with a magnifier or a phone camera at maximum zoom, against a dark background with light from the side. Round gas bubbles — perfectly spherical, sometimes in trails — are a signature of glass. Natural quartz has inclusions too, but they are angular crystals, needles, fluid pockets with irregular shapes, or planes of tiny "fingerprint" inclusions, not round bubbles.
Swirl marks. Glass that was poured or moulded often shows curved, flowing lines inside, like stirred syrup. Quartz has no flow lines; its internal features are straight, angular or planar.
Mould seams. Moulded glass beads and spheres may carry a faint raised line where the two halves of the mould met, and the drill hole of a moulded bead can have a slightly rounded, glassy rim. Quartz beads are cut and drilled; the hole edge is sharp.
Temperature. Quartz conducts heat away from the skin faster than glass, so a quartz sphere or bead feels distinctly cold and stays cold longer; glass warms quickly in the hand. This is a real effect, but it needs a comparison piece to be reliable.
Hardness. Quartz is 7 on the Mohs scale and scratches ordinary glass (5.5–6). A quartz point drawn firmly across a scrap of window glass leaves a scratch; a glass "point" does not. Do this only with a piece you are willing to risk, only on scrap glass, and never on the stone itself — some hard glasses and other minerals also scratch glass, so a scratch proves the piece is at least as hard as glass, not that it is quartz.
The doubling test (for clear pieces). Place a clear quartz sphere or a thick clear bead on a printed line and look through it. Quartz is doubly refractive — it splits light into two rays — and in a sphere or a thick piece a single line can appear faintly doubled or fuzzy along some viewing directions. Glass shows one clean line from every direction. The effect in quartz is subtle (its birefringence is small) and easiest to see in larger pieces.
What the laboratory measures
Refractive index. Quartz reads 1.544–1.553 on the refractometer, two readings, always. Glass gives one reading anywhere from about 1.47 (borosilicate) to 1.7 or higher (lead glass) — the number depends on the recipe, but it is a single reading, and it is rarely exactly 1.54–1.55.
Polariscope. Between crossed polarisers quartz blinks light and dark every 90 degrees; a quartz sphere shows a distinctive "bull's-eye" interference figure that is diagnostic on its own. Glass stays dark or shows an irregular, patchy strain pattern that never resolves into the regular blinking of a crystal.
Specific gravity. Quartz is 2.65. Soda-lime glass is about 2.5, lead glass 3.0–4.0. A large clear sphere weighed and measured gives a quick density check even without immersion.
Microscope. Bubbles, swirls and devitrification patches in glass; angular inclusions, needles, fluid inclusions, growth zoning and "veils" in quartz.
Any one of the first three tests is conclusive. Together they take a few minutes.
Special cases worth knowing
Synthetic quartz. Quartz is grown hydrothermally in large quantities for the electronics industry, and some of it is cut into gems and beads. Synthetic quartz is quartz — same RI, SG and hardness — and passes every test above. Separating it from natural quartz needs microscopic examination for the seed plate and characteristic growth zoning, and in very clean material can be difficult. Most synthetic quartz on the market is coloured (citrine-yellow, amethyst-purple, green, blue), because that is where the money is; large clear natural quartz is abundant enough that synthesising it for gems is rarely worthwhile.
Reconstituted or "fused" quartz. Quartz powder melted into a glass. Chemically silica, but amorphous — it is glass and tests as glass.
Coloured glass sold as amethyst, citrine, aventurine and more. Everything above applies. Purple glass has an RI of about 1.5 and one reading; amethyst reads 1.544–1.553 with two. See Amethyst: natural, synthetic and glass and Goldstone is man-made.
"Crystal" glassware and chandelier crystal. Lead glass, always. The word is a trade term with no mineral meaning.
Sphatik malas
Sphatik (clear quartz) malas are a special case because a mala is 108 small beads, and testing each on a refractometer is impractical. The laboratory tests a sample of beads for RI and polariscope reaction, weighs the mala, and examines beads under magnification for bubbles and mould features. Malas made of glass usually fail on the first bead. Sphatik (clear quartz): testing malas and spheres goes into the specifics.
Frequently asked questions
Is "crystal" glass real crystal?
No. Crystal glassware and crystal beads are glass — usually lead glass, chosen because its high refractive index makes it sparkle. It has no crystalline structure. In mineral terms it is not crystal at all.
Do bubbles always mean glass?
Round, spherical bubbles do. Quartz can contain fluid inclusions that hold a gas bubble inside a liquid pocket, but these pockets are irregular in shape and the bubble sits inside them; they do not look like the free, perfectly round bubbles in glass.
My clear sphere has no bubbles or inclusions at all. Quartz or glass?
Either is possible: fine glass can be flawless, and some natural quartz is very clean. A perfectly clean piece needs the polariscope or refractometer — the bull's-eye interference figure in a quartz sphere settles it in seconds.
Does real quartz feel cold?
Colder than glass, yes, because it conducts heat better. But "cold" is relative and depends on the room, your hand and the size of the piece. Use it as a hint, not a verdict.
Can a laboratory tell natural quartz from synthetic?
Usually, by growth features under the microscope. For very clean colourless material the distinction can be difficult and a laboratory should say so. For most buyers the important line is quartz versus glass, which is never in doubt after testing.