The RockLens field guide
Quartz vs glass: how they differ and how to tell them apart
Quartz is a mineral with a crystal structure. Glass has none, even when it is made from melted quartz. Most glass is made mainly of silicon dioxide, the same compound as quartz, but its atoms aren’t arranged in a regular pattern. Quartz is also harder: 7 on the Mohs scale, while ordinary glass is about 5 to 6. Even so, shine, bubbles or one scratch rarely tell you which you have. See USGS on quartz hardness and GIA on the hardness of glass.
What is the difference between quartz and glass?
In quartz, silicon and oxygen atoms repeat in a regular crystal pattern. In glass they don’t. Penn State’s materials science course calls glass a noncrystalline, or amorphous, form of silicon dioxide. The course also explains that ordinary glass has added ingredients such as sodium, which lower its melting temperature and make it easier to shape.
Polished quartz and glass can look the same in a photo, so use the clues below to decide what to check next.
Is quartz glass the same as quartz?
No. Quartz glass is glass made of pure silicon dioxide. You may also see it called fused quartz, fused silica or silica glass. The Getty’s Art & Architecture Thesaurus files all of these names under fused silica glass, which it describes as 100% silicon dioxide.
To make fused quartz, manufacturers melt high-purity natural quartz crystals at about 2,000 °C (3,600 °F), according to a Pacific Northwest National Laboratory report. Other fused silica starts from high-purity sand or a silicon-rich chemical. In each case the result is an amorphous glass with the chemistry of quartz but not its crystal structure. A piece sold under any of these names is glass, not a natural quartz crystal.
What to look for
| Observation | What to check | How to interpret it |
|---|---|---|
| Clear or colored, with a glassy shine | Photograph the whole object and any internal features. | A glassy appearance alone leaves both possibilities open. |
| Round features inside | Use magnification to see whether they sit inside a larger cavity. | Bubbles can occur in glass and in fluid inclusions in quartz. |
| A scratch-test result | Check the test material and whether a real groove formed. | Hardness helps narrow the possibilities but cannot establish origin. |
| A label says “quartz glass” or “fused quartz” | Ask whether it is sold as glass or as natural quartz. | Both names describe glass made of silicon dioxide. |
| A seller calls it “quartz” | Ask what testing supports the identification. | A seller’s description is a starting point to check. |
What bubbles can tell you
At a gem show, a GIA staff member looked at beads sold as “cherry quartz” through a loupe and saw many round gas bubbles, a sign that they might be glass. Laboratory tests, including refractive index and specific gravity, confirmed that they were. The case is in the Spring 2003 Gems & Gemology lab notes.
Natural quartz can hold a bubble too. Mobile Inclusions in a Decorative Quartz Object, a report published in GIA’s journal, describes a polished, barrel-shaped piece with a gas bubble that moves inside a fluid-filled cavity. Standard gemological tests and inclusions typical of natural growth identified it as quartz, and laboratory spectroscopy confirmed the result. Neither identification rested on the bubble.
Is quartz harder than glass?
Yes. The USGS mineral gemstone list gives quartz a Mohs hardness of 7, and GIA puts glass at about 5 to 6, so quartz can scratch ordinary glass. That doesn’t make “it scratches glass” a test for quartz. As GIA points out, anything at least as hard as glass can scratch it, so other tests are needed to confirm what a stone is.
The hardness of a household test object may be unknown, and a powder trail can look like a scratch. Wipe the surface and check for a groove before interpreting the result. Polished or coated surfaces can make this harder. If the scale is new to you, our explainer on Mohs hardness lists the ten reference minerals and what the numbers mean.
Leave jewelry and valuable specimens unmarked. On an ordinary sample, follow a hardness kit’s instructions and use a small, inconspicuous spot. Avoid breaking glass or handling sharp edges. Even a clear hardness result needs to be considered alongside the other properties.
How to examine an unknown piece
- Keep the background details. Save where it was found, whether it was bought, and the exact seller description.
- Take photos from several angles. Use diffuse light and a plain background. Include the full object, an edge, and close details without filters.
- Write observations separately from conclusions. Write down what you can see, such as “round feature near the edge,” so you can compare it with reference examples later.
- Ask a gemologist when the answer matters. They can measure properties such as refractive index and specific gravity, as in the two cases above, and choose further instrument tests if those aren’t enough. Get an identification before spending a substantial amount or making an authenticity claim.
Can an app tell whether quartz is genuine?
A photo app can suggest likely matches. Confirming natural origin requires further testing, as the examples above show. Keep the app’s suggestion with your notes until you have enough information to confirm it. The same goes for a photo search; see what a photo search like Google Lens can tell you.
If the piece is much softer than quartz, compare quartz and calcite. You can find both comparisons in our identification guides.