The group
The gem feldspars are interesting for a reason unusual among gems: most of their beauty is not colour at all but interference. Light bounces between microscopic layers inside the stone and comes back as a sheen that moves when you move.
Moonstone shows adularescence, a soft blue float caused by alternating lamellae of two feldspars. Labradorite shows labradorescence, the sharp spectral flash from twinning planes; the Finnish material with a full spectrum is sold as spectrolite. Sunstone shows aventurescence from platelets of copper or hematite.
The practical caution is the same across the group: perfect cleavage in two directions at only 6 to 6.5 hardness. Feldspars chip. They are excellent in pendants and earrings and in bezel-set rings, and a poor choice for an unprotected daily ring.
One market caution: copper-diffused andesine caused a significant scandal and treated material is still circulating with claimed natural origins. Origin claims on red and green feldspar deserve a report.
Every feldspar compared
The whole group side by side. Where the constants overlap, the species cannot be named from a refractometer alone — which is exactly why the trade names mislead.
| Variety | Species | Colour | Mohs | RI | SG | Guide price |
|---|---|---|---|---|---|---|
| Adularia | Feldspar group (alkali) | colourless | 6–6.5 | 1.518–1.536 | 2.54–2.61 | Alpine crystals 20–300 USD per specimen; sheen cabochons 5–60 USD |
| Albite | Albite (plagioclase feldspar) | colourless | 6–6.5 | 1.527–1.538 | 2.60–2.65 | $1–20/ct |
| Amazonite | Microcline feldspar | teal | 6–6.5 | 1.522–1.530 | 2.56–2.58 | $1–20/ct |
| Andesine | Andesine (plagioclase feldspar) | red | 6–6.5 | 1.543–1.554 | 2.65–2.69 | $5–50/ct; heavily discounted since the diffusion scandal |
| Anorthite | Feldspar group (plagioclase) | colourless | 6–6.5 | 1.575–1.588 | 2.74–2.77 | Faceted stones typically 20–150 USD/ct because of rarity, not beauty |
| Anorthoclase | Feldspar group (alkali) | colourless | 6–6.5 | 1.518–1.534 | 2.56–2.62 | 15–100 USD/ct |
| Bytownite | Bytownite (plagioclase feldspar) | yellow | 6–6.5 | 1.570–1.585 | 2.72–2.75 | $5–60/ct |
| Dalmatian Stone | Quartz-feldspar porphyry - a fine-grained igneous rock with dark phenocrysts; not a jasper and not chalcedony | colourless | 6-7 | 1.54 approx. (spot) | 2.60-2.75 | $2-6 per kilogram rough; bead strands $3-15 |
| Eudialyte-Bearing Agpaitic Rock | Layered agpaitic nepheline syenite in which pink eudialyte forms the ornamental component, with black arfvedsonite and white feldspar | pink | 5-6 | 1.59-1.64 (eudialyte) | 2.70-3.10 | $5-40 per kilogram rough; cabochons $15-90; fine Greenlandic kakortokite slabs higher |
| Feldspar (Industrial) | Alkali and plagioclase feldspars used as a flux to lower melting temperature in glass and ceramics - the single largest industrial use of any silicate after quartz | colourless | 6-6.5 | 1.518-1.539 | 2.55-2.76 | $50-150 per tonne for standard glass and ceramic grades; low-iron and fine-milled grades $180-400 per tonne |
| Gneiss | High-grade metamorphic rock with a banded segregation of light feldspar-quartz and dark mica-amphibole layers | colourless | 6-7 | Opaque | 2.65-2.85 | $40-250 per square foot installed; exotic Brazilian gneiss at the top end |
| Granite | Coarse-grained felsic intrusive igneous rock of quartz, alkali feldspar and plagioclase; in the stone trade the word also covers gabbro, anorthosite, gneiss and charnockite | pink | 6-7 (quartz-controlled) | Opaque | 2.60-2.80 | $30-150 per square foot installed; exotic Brazilian and Indian stones $100-400 per square foot |
| Kaolin | A clay rock dominated by kaolinite, formed by weathering or hydrothermal alteration of feldspar in granite and arkose | colourless | 2-2.5 | 1.553-1.570 | 2.16-2.68 | $80-200 per tonne for filler grades; premium coating, calcined and pharmaceutical grades $250-600 per tonne |
| Labradorite | Plagioclase feldspar | blue | 6–6.5 | 1.559–1.573 | 2.68–2.72 | $1–50/ct |
| Larvikite | Monzonite rock (ternary feldspar) | blue | 6–6.5 | 1.52–1.57 (feldspar components) | 2.65–2.75 | Sold by the slab; cabochons $1–10/ct |
| Microcline | Feldspar group | green | 6–6.5 | 1.514–1.539 | 2.55–2.63 | Rough 2–20 USD/kg; good amazonite cabochons 5–40 USD |
| Moonstone | Orthoclase / albite feldspar | colourless | 6–6.5 | 1.518–1.526 | 2.56–2.59 | $2–150/ct |
| Nebula Stone | An unusual alkaline igneous rock of quartz, aegirine, riebeckite and anorthoclase with green orbs in a black ground; a registered trade name from a single deposit | green | 6-7 | 1.54 approx. (spot) | 2.65-2.85 | $30-150 per kilogram rough; cabochons $25-200 |
| Oligoclase | Feldspar group (plagioclase) | colourless | 6–6.5 | 1.538–1.550 | 2.62–2.67 | 2–25 USD/ct for clean faceted material |
| Oregon Sunstone | Labradorite (plagioclase feldspar) | red | 6–6.5 | 1.563–1.573 | 2.68–2.72 | $20–500/ct; fine red/green dichroic material higher |
| Orthoclase | Orthoclase (feldspar group) | yellow | 6–6.5 | 1.518–1.526 | 2.55–2.59 | $20–150/ct |
| Perthite | Feldspar intergrowth | colourless | 6–6.5 | 1.518–1.539 | 2.55–2.62 | Slabs 2–15 USD/kg; cabochons 5–40 USD |
| Rainbow Lattice Sunstone | Orthoclase feldspar with oriented inclusions | yellow | 6–6.5 | 1.518–1.526 | 2.55–2.60 | Genuine cabochons 200–2,000 USD/ct |
| Rainbow Moonstone | Labradorite (plagioclase feldspar) | colourless | 6–6.5 | 1.559–1.568 | 2.68–2.72 | $2–40/ct |
| Sanidine | Sanidine (feldspar group) | colourless | 6–6.5 | 1.518–1.524 | 2.56–2.62 | $5–50/ct |
| Spectrolite | Labradorite (plagioclase feldspar) | blue | 6–6.5 | 1.559–1.570 | 2.68–2.72 | $2–40/ct |
| Sunstone | Feldspar | orange | 6–6.5 | 1.537–1.547 | 2.62–2.67 | $5–300/ct |
| Sunstone (Oligoclase) | Oligoclase (plagioclase feldspar) | orange | 6–6.5 | 1.537–1.547 | 2.62–2.65 | $1–25/ct |
| Unakite | Altered granite (epidote + orthoclase + quartz) | green | 6–7 | 1.52–1.77 (variable by phase) | 2.85–3.20 | $0.20–3/ct |
Questions
What causes the blue sheen in moonstone?
Adularescence — light scattering between alternating microscopic layers of two different feldspars inside the stone. It is an optical effect, not a body colour, which is why it moves as the stone moves.
Is labradorite the same as spectrolite?
Spectrolite is a trade name for labradorite from Ylämaa in Finland that shows the full spectrum rather than one or two colours. Same species, exceptional material.
Is feldspar durable enough for jewellery?
For pendants and earrings, yes. For a ring worn daily, only in a protective setting — at 6 to 6.5 with perfect cleavage in two directions, feldspar chips more readily than its hardness suggests.
What is andesine and why is it controversial?
A plagioclase feldspar. Red and green material sold as natural Tibetan andesine was largely copper-diffused, and treated stones are still in circulation. Buy it on a laboratory report or not at all.
Looking for feldspar?
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