aphanite
Certain dark igneous rocks having grain so fine that the individual crystals cannot be seen with the naked eye.
aphanite: dark rock too fine-grained to see crystals by eye
Aphanite is a dark igneous rock, typically basalt or diabase, in which crystals are so small that you cannot distinguish individual mineral grains without magnification. The rock cooled quickly, usually at or near the surface, which prevented large crystals from forming. Obsidian and basalt are common aphanites; they appear uniform and glassy or dull to the naked eye even though they are crystalline at the microscopic scale.
The term comes from the Greek aphanes, meaning invisible or obscure, which directly describes the invisible crystal structure. Under a thin section viewed through a microscope at 100x magnification or higher, you will see mineral phases: pyroxene, feldspar, and magnetite are typical in basaltic aphanites. The crystal size is usually less than 1 millimeter across, sometimes much finer.
Aphanite is distinguished from porphyritic rocks, which show both fine and coarse crystals, and from pegmatite, which has exceptionally large crystals. It contrasts with rocks like granite, where grain is coarse enough that quartz, feldspar, and mica are visible to the unaided eye. Aphanite also differs from volcanic glass: obsidian is technically amorphous rather than crystalline, though it is sometimes classed alongside true aphanites because it also appears structureless at hand-lens scale.
In quarrying and aggregate production, aphanites like basalt and trap rock are valued for their hardness and durability. They are crushed for road base, railroad ballast, and concrete aggregate. Because the crystals are tightly bonded and uniform in size, aphanites tend to break evenly and produce fewer fines than rocks with large grain variation. Brittleness and the formation of conchoidal fracture patterns are typical during crushing.
Drilling and blasting crews recognize aphanites as relatively consistent to work with: absence of visible crystal boundaries means fewer planes of weakness, so the rock does not break preferentially along grain. However, the fine grain also means less permeability and slower water infiltration, which can affect hole stability and powder placement in blastholes.