Quick answer: the fastest reliable checks are (1) look for round bubbles with a 10x loupe: any spherical bubble means glass; (2) check for fine horizontal growth lines on the prism faces, which natural quartz has and molded glass does not; and (3) a hardness test: quartz (Mohs 7) scratches ordinary glass, while glass cannot scratch quartz. Temperature, weight, and UV give supporting evidence but do not prove anything alone.
Why this question matters
Clear quartz is common and inexpensive, but glass is cheaper still, and molded glass “points,” spheres, and clusters are sold as quartz all over the internet. Some fakes are glass outright; others are melted or reconstituted quartz bound with resin. The good news is that natural quartz has a set of physical properties glass struggles to copy. (Many other species get faked too; see What Crystals Are Most Commonly Faked (and How to Spot Them).)
The 7 tests
1. The bubble test (definitive when positive)
Put a strong light behind the piece and look through it with a 10x loupe. Rockhounding.org’s guide puts it plainly: any spherical bubble is a definitive indicator of glass. A bubble-free stone is not automatically quartz, because good glass can be clean, so keep testing.
2. Growth lines (striations)
Natural quartz crystals usually show fine, parallel horizontal lines across the six prism faces, a record of how the crystal grew. Molded glass shows smooth faces or mold seams instead. Look at a natural point such as our Clear Quartz Crystal Point under magnification and you will see them.
3. Hardness
Quartz is 7 on the Mohs scale; window glass is about 5.5. Quartz scratches glass, and glass will not scratch quartz. Rockhounding.org adds a useful tip: run a fingernail across the mark afterward, because a streak of powder can wipe away and fool you, while a real groove catches the nail. Test on an inconspicuous spot. (Refresher: The Mohs Scale.)
4. The cold-touch test
Quartz conducts heat about four times faster than soda-lime glass, according to Rockhounding.org, so it feels cool and keeps feeling cool. Glass held for about 30 seconds warms toward body temperature. A refinement: chill both on an ice cube for 30 seconds and touch each to your inner wrist; quartz stays colder longer. Useful, but supporting evidence only.
5. Inside the stone
Natural quartz often contains growth veils, phantoms, needles of rutile, fluid inclusions, and irregular healed fractures. Glass tends to look uniform inside. A phantom, like the ghost outlines in our Milky Phantom Quartz Cluster — New Hampshire, is something glass simply does not produce. (More on those in Phantom Quartz: The Ghost Crystal Hiding Inside Another Crystal.)
6. Weight (specific gravity)
Quartz has a specific gravity of about 2.65; soda-lime glass is about 2.5, while leaded “crystal” glass can be heavier. You can measure this with a kitchen scale and a cup of water, but the gap is small, so Rockhounding.org treats it as supporting evidence only.
7. UV light
Strong blue-white fluorescence under UV is a common glass signature. Weak or no glow fits quartz but is not definitive. (Why minerals glow: Fluorescence: Why Some Rocks Glow Under UV Light.)
What these tests can’t tell you
None of the home tests reliably separates natural quartz from lab-grown quartz, which is real quartz with the same hardness and heat behavior. Rockhounding.org notes there is no dependable field test for that; it takes laboratory methods. For natural crystal points, a stated locality and natural growth features (striations, contact marks, inclusions, damage consistent with a pocket) are your best evidence, which is why we list where each piece was found.
A note on fracture
Both quartz and glass break with curved, shell-like conchoidal fractures, so a chipped edge does not distinguish them. That shared trait is explained in Conchoidal Fracture: Why Quartz and Glass Break into Curves.
Quick FAQ
Does real quartz have bubbles?
Natural quartz can contain fluid inclusions with tiny gas bubbles, but these sit inside irregular or angular fluid cavities. Perfectly round, free-floating spherical bubbles are a classic sign of glass.
Is quartz colder than glass?
Yes, it usually feels colder and stays cool longer because it conducts heat roughly four times faster than ordinary soda-lime glass.
Will quartz scratch glass?
Yes. Quartz is about Mohs 7 and window glass about 5.5, so quartz scratches glass, but glass will not scratch quartz.
