Identification guide · Igneous Rocks
How to Identify Porphyry
Porphyry is recognized by its two distinct crystal sizes: scattered, often blocky phenocrysts of feldspar, quartz, or other minerals set within a much finer, sometimes glassy groundmass. Color and hardness vary with composition, from pale felsic porphyry to dark mafic varieties, so hardness testing must target individual phenocrysts rather than the whole rock. There is no single porphyry mineral or streak, and it is best identified by texture, not composition, distinguishing it from evenly grained granite or basalt.
Porphyry identification checklist
| Color | Purple, Red, Gray, Green | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Dull to vitreous | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Opaque | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 5–7 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Fracture | Conchoidal to uneven | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.6–2.8 | Ordinary rock-like heft (specific gravity about 2.7), similar to quartz. |
Extra tests for Porphyry
Glass check. Porphyry shares a glassy luster and conchoidal fracture with man-made glass. Glass sits near Mohs 5.5 and often contains round gas bubbles, so a hardness test and a hand lens separate them.
Porphyry look-alikes and how to tell them apart
Both sit near Mohs 5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Porphyry vs Feldspar →
Quartz is harder (Mohs 7) than Porphyry (Mohs 5–7), so Quartz will scratch Porphyry and not the other way round. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. Porphyry vs Quartz →
Both sit near Mohs 5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Porphyry vs Andesite →
Both sit near Mohs 5–7, so a scratch test alone will not separate them. Lean on luster, streak and heft instead.
Porphyry identification FAQ
How can you tell if a rock is Porphyry?
Check the color (purple, red, gray, and green), the dull to vitreous luster, a Mohs hardness of 5–7. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Porphyry?
Porphyry has a Mohs hardness of 5–7. It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will.
What is the difference between Porphyry and Feldspar?
Porphyry (Mohs 5–7, dull to vitreous luster) and Feldspar (Mohs 6–6.5, vitreous to pearly luster) look alike, but hardness, streak and heft usually separate them. See the full Porphyry vs Feldspar comparison for a side-by-side table.
Is Porphyry worth anything?
Entry-level Porphyry specimens sell for around $1, typical collector pieces about $10, and premium examples $75 or more depending on size, quality and locality.
What color is Porphyry?
Porphyry is most often purple, red, gray, and green. Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
