Rock Crystal or Glass: Reliable Clues, Without Dangerous Tests

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Rock crystal (quartz), glass, lead crystal or synthetic quartz: what each material is, the useful clues, and the limits of tests done at home.

Key points

Rock crystal	Colourless quartz (silicon dioxide), hardness 7, density about 2.65
Ordinary glass	Hardness about 5.5; density about 2.4 to 2.6; amorphous, with no crystal structure
Crystal (lead glass)	European directive 69/493/EEC: at least 24% lead oxide and a density of at least 2.9 for the term “lead crystal”
Most useful clue	Round bubbles and swirl marks in glass; irregular inclusions in quartz
Pitfalls	Synthetic quartz, glass without bubbles, “crystal” balls made of glass, treated quartz
Safe test	Loupe, weight, comparison, reading the description, a laboratory if the value justifies it

Four materials behind the word “crystal”

The word “crystal” causes confusion. In everyday use it may refer to natural material, luxury glassware, or simply any object that sparkles. Here are the four materials to tell apart.

Rock crystal is colourless, transparent quartz. It is crystallised silicon dioxide (SiO2), with a Mohs hardness of 7 and a density of about 2.65. It forms in cracks or cavities of rocks, often with heat and hot water carrying dissolved silica. Deposits are numerous: the Alps, Brazil (Minas Gerais), Madagascar, Arkansas (United States).
Ordinary glass is an amorphous material, made by melting sand, soda and lime. Its hardness is about 5.5 and its density 2.4 to 2.6, very close to that of quartz. This is why weighing alone almost never settles the question.
“Crystal” in the glassmaking sense is glass containing lead oxide (or sometimes other oxides). European directive 69/493/EEC sets the thresholds: for the term “lead crystal”, at least 24% lead oxide and a density of at least 2.9. It is therefore heavier than quartz. It is not a mineral.
Synthetic quartz is manufactured in a laboratory, in hot water under pressure (the hydrothermal method). It is real quartz, with the same composition and structure, but not natural: it must be described as “synthetic”.

The table of clues

Clue	Rock crystal	Glass
Bubbles	No round bubbles; sometimes irregular liquid inclusions	Round or elongated bubbles, sometimes tiny and aligned
Internal streaks	None; rather planes, veils or “feathers”	Sinuous streaks, a look of “honey being stirred” (traces of mixing)
Inclusions	Fine needles, veils, small cavities with liquid, sometimes rainbow effects in cracks	No natural inclusions, or bubbles
Growth lines	Fine regular lines, perpendicular to the axis, on the faces of a rough crystal	Moulded surface with no lines, or imitated lines
Fracture	Conchoidal (shell-like) on a chip	Also conchoidal: no clue
Temperature to the touch	Stays cool for longer (conducts heat better); but the effect depends on the starting temperature	Less cool; weak clue
Weight	Density about 2.65	Very close for ordinary glass; heavier for lead crystal

Modern glass can be perfect and free of bubbles. The absence of flaws therefore proves nothing, whereas the presence of round bubbles is an excellent sign of glass.

Simple, safe tests

Look for bubbles under a loupe (10x magnification). Light the piece from the side, against a dark background. Round bubbles or sinuous streaks point towards glass.
Observe the inclusions. Clearly visible, irregular, non-spherical inclusions are more likely natural. But this is only a clue: a quartz can be very pure, and some glass is designed to imitate inclusions.
Examine a rough crystal. The faces of a real quartz crystal carry fine lines perpendicular to the axis of the prism, and the tip is a six-faced pyramid. A “crystal” point that is perfectly smooth and moulded is suspect.
Try the hand test. Place the piece against your cheek or wrist, then compare it with a piece whose nature is known: quartz feels cooler, for longer. This test is subjective: if the room is warm or the piece has just come out of a pocket, it gives misleading results.
Looking at text through a sphere does not separate the two materials: glass and quartz both invert the image.
If the piece is faceted, a gemmologist's eye or a refractometer can distinguish the double reflection of quartz from the single image of glass. This is a professional examination.

Avoid. The scratch test (quartz scratches ordinary glass, but some hard glasses, such as borosilicate, come close) leaves a mark on the piece. Do not use a blade, a flame or acid.

What these tests cannot prove. None of them separates natural rock crystal from synthetic quartz. Only a laboratory can detect the signs of artificial growth. Nor do they rule out a modern glass without flaws.

Spheres, points and objects: what you really find

Crystal balls are most often made of glass, lead crystal or, more rarely, quartz. Balls of genuine rock crystal, without bubbles, are expensive because they require a large block of pure material. A large sphere of perfectly clear rock crystal sold at the price of glass is therefore a warning sign. For details, see our notes on spheres and how to choose a crystal ball.

Points carved from a single block of quartz are common. Moulded glass points also exist: they show very regular edges and a perfectly flat base. Raw quartz crystals from the same batch are almost always a little different from one another; crystals that are strictly identical should raise your attention.

Finally, some quartz crystals are surface-treated: a metallic coating giving a rainbow sheen (“aura” quartz), artificial colouring, and so on. These are genuine quartz, but they are no longer “rock crystal” in the strict sense, and the coating wears away over time.

Words that cause confusion

Word or mention	Possible meaning
Crystal	Lead glass, or the everyday sense of an “object made of quartz”; needs clarifying
Bohemian crystal	Bohemian glassware, lead or non-lead depending on the case; it is not quartz
Rock crystal	Natural colourless quartz
Synthetic crystal, synthetic quartz	Quartz manufactured in a laboratory, with the same composition as natural quartz
Clear quartz, hyaline quartz	Other names for rock crystal
Arkansas crystal, Herkimer diamond	Place names for particularly clear quartz; Herkimer is in fact a quartz, not a diamond

For general rules on reading labels, see how to read a label.

What to ask and what to remember

Before buying, ask: the exact material (natural quartz, glass, lead crystal, synthetic quartz), the weight, and any mention of a treatment. For a valuable piece (a large sphere, a collector's point), a laboratory certificate is the only real safeguard.

Keep in mind that, for symbolic or decorative use, a beautiful glass honestly presented is a lovely object; what matters is that the material is named correctly. That is the commitment we make on our product pages (see our commitments). For caring for a quartz, consult the cleansing guide and the rock crystal profile.

Frequently asked questions

How can I tell whether a crystal is glass?

Look under a loupe for round bubbles or sinuous streaks: these are good signs of glass. Modern glass can nevertheless be perfect, and synthetic quartz is real quartz. If in doubt about a valuable piece, a laboratory is the only safe recourse.

Is rock crystal colder to the touch than glass?

Often a little, since quartz conducts heat better, but the test is subjective and depends on the starting temperature. It is not enough to reach a conclusion.

Is lead crystal rock crystal?

No. It is glass containing lead oxide, which gives it high brilliance. European directive 69/493/EEC sets thresholds for the name “crystal”: at least 24% lead oxide for lead crystal.

Is a very clear crystal ball necessarily glass?

Not necessarily, but flawless balls of genuine rock crystal are rare and expensive. If the price is that of glass, the ball very probably is glass.

Can I scratch a crystal to find out what it is?

Some people do, but this test leaves a permanent mark, and some hard glasses resist it. It is best avoided on a piece you want to keep.