Hardness 7, no cleavage, a glassy conchoidal break and no fizz in acid. The mineral most finds turn out to be.
Every continent — the commonest mineral in the crust6 field tests
If you have picked up a hard, glassy, roughly translucent stone and want to know what it is, quartz is the most likely answer before you have looked at it. Silicon dioxide makes up something like a tenth of the crust by mass and turns up in almost every geological setting there is, which is why a beach, a riverbed and a driveway all produce it.
That commonness is the useful thing about it. Quartz sits at 7 on the Mohs scale, which means it scratches window glass and a steel knife and is scratched by almost nothing you own. It has no cleavage at all, so it breaks in curved, shell-like conchoidal fractures rather than along flat planes. And it does not react to acid. Those three facts together eliminate most of what quartz gets confused with in about a minute.
The reason people misidentify it is that quartz is a shapeshifter by colour and habit. Purple is amethyst, yellow is citrine, pink is rose quartz, brown is smoky, banded is agate, fibrous is chalcedony, and micro-crystalline opaque red is jasper — all of them the same mineral with the same hardness and the same fracture. Learn the species and you have learned two dozen names at once.
These are measured properties rather than appearances, which is exactly why they work. Every one of them can be checked at a kitchen table, and none of them can be faked by a photograph.
The specific gravity figure matters more than it looks. At 2.65 quartz feels ordinary in the hand — noticeably lighter than pyrite, garnet or hematite, and noticeably heavier than amber or opal. Heft is a real diagnostic and most people never learn to use it.
| Property | Quartz | What that rules out |
|---|---|---|
| Mohs hardness | 7 | Calcite (3), fluorite (4), apatite (5), feldspar (6) — all softer |
| Streak | White (or none — it is harder than the tile) | Any metallic mineral |
| Specific gravity | 2.65 | Pyrite 5.0, garnet 4.0, hematite 5.3, opal 2.1 |
| Cleavage | None | Feldspar, calcite, fluorite — all cleave on flat planes |
| Fracture | Conchoidal — curved, glassy, shell-like | Anything that breaks flat |
| Crystal system | Trigonal — six-sided prisms with pointed terminations | Cubic pyrite, octahedral fluorite |
| Reaction to dilute acid | None | Calcite, malachite, aragonite — all effervesce |
| Lustre | Vitreous (glassy) on fresh faces | Metallic and earthy minerals |
A stone that is hardness 7, has no cleavage, breaks conchoidally and does not fizz is quartz or one of its varieties in the overwhelming majority of cases.
Ordered by how much each one proves. The decisive tests are worth your first two minutes; the supporting ones only matter once those agree.
Quartz at 7 scratches a glass plate (5.5) easily and a steel blade cannot mark it. That single pair of tests separates quartz from every common lookalike except topaz and beryl, which are rare enough that you are not holding one. Do the scratching on an inconspicuous spot and wipe away the powder before you decide — a soft mineral leaves a streak of its own dust that looks exactly like a scratch until you clean it.
Find a chipped or broken edge and hold it to the light. Quartz has no cleavage whatsoever, so it breaks in smooth curved surfaces like the inside of a shell, often with faint concentric ripples. Feldspar, calcite and fluorite all break along flat mirror-like planes at characteristic angles instead. If you can see two flat faces meeting at a consistent angle across several fragments, it is not quartz.
Nothing should happen. Quartz is a silicate and completely inert to weak acid. If the stone fizzes, even faintly — watch it under a hand lens for slow bubbles rather than expecting foam — it contains carbonate and you are holding calcite, aragonite, marble or malachite instead. This is the cheapest test in the book and it eliminates a whole family in ten seconds.
Hold a piece of quartz in one hand and a similar-sized piece of anything metallic-looking in the other. At 2.65, quartz feels unremarkable. Pyrite at about 5.0 feels close to twice as heavy for the same volume, and so does hematite; garnet feels obviously dense; opal feels suspiciously light. Heft is a proper diagnostic property and the fastest one available once you have handled a few dozen stones.
A well-formed quartz crystal is a six-sided prism ending in a six-faced point, and the prism faces usually carry fine horizontal striations across them. That combination is close to conclusive. Beware, though — most quartz occurs as massive lumps with no crystal shape at all, so an absent crystal form proves nothing either way.
Quartz is colourless when pure and takes on almost any colour from trace impurities and radiation damage. Purple, yellow, pink, brown, grey, green, banded and milky are all quartz. Meanwhile amethyst-purple also occurs in fluorite, and rose-pink in calcite and rhodonite. Colour narrows a stone to a variety only after the species is settled, never before.
Nothing is faked as quartz, because quartz is worth almost nothing by weight. What happens instead is the reverse: quartz is the thing other stones are faked out of, and manufactured glass is sold as quartz to people who wanted a crystal.
Glass is the only real trap. It is close in appearance, sits at about 5.5 on the Mohs scale so it will not scratch a knife blade, and it very often contains round trapped bubbles — which is the giveaway, because a gas bubble is a thing only a melt produces. Natural quartz has angular inclusions, healed fractures and internal veils, but essentially never a perfect sphere of air.
Each of these is checked differently. The ones with their own page have enough that is specific to them to be worth reading separately.
White and opaque from countless fluid inclusions. The commonest quartz on earth.
Read within the checks aboveSpecimen figures rather than gem figures, for material in honest condition. Most finds are worth very little, and saying so plainly is more useful than encouraging you.
| Material | What it typically fetches | Note |
|---|---|---|
| Common milky or vein quartz | Nothing | Abundant beyond any commercial use |
| Tumbled rose or smoky quartz | $2 – 10 a piece | Sold by weight, not by specimen |
| Clear quartz point, hand-sized | $5 – 40 | Clarity and termination quality set it |
| Fine cabinet crystal, named locality | $100 – 1,000+ | Locality and aesthetics, not the mineral |
| Quartz as a faceted gem | A few dollars a carat | Amethyst and citrine excepted, barely |
Quartz is priced almost entirely on aesthetics and provenance rather than on the material, which is why a museum-grade crystal and a driveway pebble are the same mineral at a ten-thousandfold difference.
Lovenhall StoneThe tests tell you what it is. The instrument reads a photograph and offers a name to start from — then hands you back to the tile and the coin, which are what settle it.