Variolite Identification Guide

What Variolite Looks Like

Variolite is a spotted volcanic rock - typically an altered basalt or spilite - speckled with pale, rounded spots called varioles. The varioles are light gray, greenish-white, or cream spherules (usually a few millimeters across) made of radiating fibrous feldspar and pyroxene that crystallized in the cooling lava. They sit in a darker green to greenish-gray groundmass. The overall effect is a dark, fine-grained rock peppered with paler pea-like spots; polished river cobbles show this contrast well and are popular as "leopard" or spotted stones.

Step-by-Step Field-ID Checklist

1. Look for the spots. Numerous pale, rounded varioles in a darker matrix is the signature.
2. Check spherule structure. Under a loupe, varioles show a radial/fibrous internal texture (they are crystallized spherulites, not bubbles).
3. Note the matrix. Dark green to gray fine-grained (basaltic/spilitic) groundmass.
4. Hardness test. Overall ~6 (basaltic minerals); scratches glass faintly, not scratched by steel.
5. Confirm it's solid rock, not vesicular - varioles are filled crystalline spheres, not hollow.
6. Acid test. No fizz (silicate rock), unless secondary carbonate is present.

Key Diagnostic Tests

• Mohs hardness: ~5.5-6.5 overall (feldspar/pyroxene).
• Streak: Gray to greenish (mixed rock).
• Cleavage/fracture: No rock-wide cleavage; tough, breaks across spots.
• Magnetism: Often weakly magnetic (magnetite in basalt).
• Acid: Generally no reaction.
• Density: ~2.9-3.0 g/cm3 (basaltic).

Common Look-Alikes

• Amygdaloidal basalt: Spots are gas-vesicle fillings (amygdules of quartz, calcite, zeolite) - these are infilled cavities, NOT radial spherulites; calcite-filled ones fizz in acid.
• Orbicular granite/diorite: Larger concentrically banded orbs in a coarse-grained plutonic rock, not a fine basaltic matrix.
• Dalmatian stone: Pale rock with dark spots (the reverse contrast) and a felsic, harder host.
• Leopard skin jasper: Microcrystalline silica (Mohs 7), waxy, with rounded color patches but no radial spherulite structure.
• Spotted porphyry: Has angular feldspar phenocrysts rather than rounded radial spherules.

The radial fibrous internal structure of the pale spots in a dark fine-grained mafic groundmass is the clincher.

Where It Is Found

Variolite occurs in altered basaltic lavas and pillow-lava margins (where rapid cooling formed spherulites). Classic localities include the Alps (the Durance River, France, source of the historic "galets de la Durance"), Scotland, Norway, and various spilite/greenstone belts worldwide; it is often collected as river cobbles.