Variolite
Variolitic mafic volcanic rock (spherulitic basalt/diabase)
A mafic volcanic rock speckled with pale spherical 'varioles,' typically formed in rapidly chilled basaltic pillow lavas.
- Mohs hardness
- 5-6
- Color
- Dark gray-green with lighter spots
- Type
- igneous
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Overview
Variolite is a mafic volcanic rock, usually basalt or diabase, distinguished by numerous small pale spherical bodies called varioles set in a darker groundmass. The varioles are radiating, spherulite-like clusters of feathery feldspar and pyroxene that form during rapid quenching of the lava.
The spotted appearance gives variolite a distinctive, ornamental look, especially in polished river pebbles. It is commonly found in the chilled margins of pillow lavas and dikes.
Variolite is a textural variety rather than a unique magma type, valued by collectors and lapidaries for its characteristic spotted patterning.
Formation & geology
Variolite forms when mafic (basaltic) magma is chilled very quickly, typically where lava erupts underwater as pillow lavas or along the rapidly cooled margins of dikes and flows. Fast cooling produces undercooling that triggers radial, spherulitic growth of feldspar and pyroxene, creating the rounded varioles within the glassy or fine-grained matrix.
Classic variolite occurs in ancient ophiolite and greenstone belts and pillow-lava sequences. Famous examples include the variolite pebbles of the Durance River in France and occurrences in Scotland, Italy, and many other regions with submarine basaltic volcanism. Some variolitic rocks are mildly altered (spilitized) seafloor basalts.
How to identify it
Look for a dark gray-green basaltic rock dotted with numerous pale, rounded spots (varioles) a few millimeters across, often standing out clearly on weathered or polished surfaces. The spots are typically lighter and slightly harder than the matrix.
Hardness is about 5-6. Distinguish variolite from amygdaloidal basalt, in which the spots are mineral-filled gas cavities (amygdales) rather than radiating crystal spherulites; varioles are solid spherulitic growths integral to the rock. Its association with pillow lavas and chilled margins is a strong contextual clue.
Uses & significance
Variolite is used mainly as an ornamental and lapidary stone: its spotted pattern makes attractive tumbled stones, cabochons, and polished specimens, and river-worn variolite pebbles are popular with collectors. It has no major industrial use beyond local aggregate.
Scientifically, variolite is important for studying rapid crystallization and ancient submarine volcanism. In metaphysical circles it is sometimes marketed as a grounding or balancing stone, though such claims are not scientifically supported. Its appeal rests chiefly on its distinctive appearance and geological interest.
Frequently asked questions
What causes the spots in variolite?
The pale spots, called varioles, are radiating spherulitic clusters of feldspar and pyroxene that grow when mafic lava is rapidly chilled.
Is variolite the same as amygdaloidal basalt?
No. In amygdaloidal basalt the spots are minerals filling old gas bubbles, whereas variole spots are solid radiating crystal growths formed during cooling.
Where is variolite found?
It occurs in pillow lavas and chilled dike margins worldwide; famous pebbles come from the Durance River in France, with other occurrences in Scotland and Italy.
Can variolite be polished?
Yes. It takes a good polish and is popular as tumbled stones and cabochons because the spotted pattern is attractive.
Variolite guides
In-depth guides for identifying, valuing, and understanding Variolite.
