Identification guide · Feldspar Group
How to Identify Feldspar
Feldspars are generally white, gray, pink, or green with a vitreous to pearly luster on cleavage faces and a white streak, and they show two cleavage directions meeting at or near 90 degrees, with hardness around 6. Plagioclase varieties typically display fine parallel striations on one cleavage face from polysynthetic twinning, while potassium feldspars like microcline instead show a cross-hatched tartan twin pattern under magnification. Crystal habit ranges from blocky to tabular. Feldspar gets confused with quartz fairly often, though quartz lacks cleavage and has a distinctly greasier luster.
Feldspar identification checklist
| Color | White, Pink, Gray, Green (amazonite), Colorless | Look at a clean, dry surface in daylight. Color zoning, banding or spots are as diagnostic as the hue. |
|---|---|---|
| Luster | Vitreous to pearly | Fresh surfaces shine like glass; a dull surface usually means weathering, so chip or wet a corner. |
| Transparency | Transparent to translucent | Hold a thin edge up to a bright light and note whether light passes through. |
| Mohs hardness | 6–6.5 | It is about as hard as window glass: a knife barely marks it, while a steel file (Mohs 6.5) will. |
| Streak | White | Drag a corner across unglazed porcelain (the back of a bathroom tile) and check the powder color. |
| Cleavage | Perfect in two directions (near 90° or 86°) | |
| Fracture | Uneven to conchoidal | Broken surfaces show smooth, curved, shell-like scoops. |
| Specific gravity | 2.55–2.76 | Ordinary rock-like heft (specific gravity about 2.655), similar to quartz. |
| Crystal system | Monoclinic to Triclinic (depending on species) |
Extra tests for Feldspar
Glass check. Feldspar 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.
Feldspar look-alikes and how to tell them apart
Both sit near Mohs 6–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Feldspar vs Albite →
Both sit near Mohs 6–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Feldspar vs Microcline →
Both sit near Mohs 6–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Feldspar vs Nepheline →
Both sit near Mohs 6–6.5, so a scratch test alone will not separate them. Lean on luster, streak and heft instead. Feldspar vs Quartz →
Feldspar identification FAQ
How can you tell if a rock is Feldspar?
Check the color (white, pink, gray, and green (amazonite)), the vitreous to pearly luster, a Mohs hardness of 6–6.5 and a white streak. Photographing it with the StoneSnap app gives a match in seconds.
What is the hardness of Feldspar?
Feldspar has a Mohs hardness of 6–6.5. 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 Feldspar and Albite?
Feldspar (Mohs 6–6.5, vitreous to pearly luster) and Albite (Mohs 6–6.5, vitreous, pearly on cleavage luster) look alike, but hardness, streak and heft usually separate them. See the full Feldspar vs Albite comparison for a side-by-side table.
Is Feldspar worth anything?
Entry-level Feldspar specimens sell for around $2, typical collector pieces about $20, and premium examples $150 or more depending on size, quality and locality.
What color is Feldspar?
Feldspar is most often white, pink, gray, and green (amazonite). Color varies with impurities and locality, so confirm with hardness and luster rather than color alone.
