igneous
Tumbled dark stone, possibly basalt or black tourmaline
Most likely fine-grained basalt (plagioclase–pyroxene), though black tourmaline (schorl, NaFe3Al6(BO3)3Si6O18(OH)4) is also possible
AI-generated identification · may be incorrectSmall rounded, matte-to-subvitreous black specimen with no visible crystals; basalt hardness ~6, specific gravity ~2.8–3.1, generally massive with poor cleavage. Tourmaline is harder (~7–7.5), vitreous, and commonly shows prismatic striations.
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Physical properties
Small rounded, matte-to-subvitreous black specimen with no visible crystals; basalt hardness ~6, specific gravity ~2.8–3.1, generally massive with poor cleavage. Tourmaline is harder (~7–7.5), vitreous, and commonly shows prismatic striations.
Formation & geological history
If basalt, it formed from rapidly cooled mafic lava, commonly in volcanic regions and oceanic crust; ages range from recent to billions of years. If schorl, it crystallized in boron-rich granitic pegmatites or hydrothermal veins.
Uses & applications
Basalt is used as aggregate, ballast, and decorative stone; schorl is mainly a collector mineral and occasional cabochon material.
Geological facts
The photograph is insufficient for confident identification; the uniformly rounded shape suggests tumbling or abrasion rather than a natural crystal form.
Field identification & locations
Check for tiny interlocking grains, which favor basalt; strong longitudinal striations or a glassier luster favor tourmaline. Common basalt sources include volcanic provinces worldwide; schorl is common in pegmatites.
