The Difference Between Basalt and Diabase
Diabase is actually a variety of gabbro which consists mainly of labradorite feldspar, augite, magnetite, and olivine. The grain size of diabase is on the smaller side. Diabase and gabbro have the same general mineral composition as basalt, but they have a larger grain size.
Basalt is identical to diabase and gabbro, but is very fine-grained. Basalt is from a melt that cooled very rapidly- in other words, on the surface of the earth or in the ocean. When basalt is broken, it leaves very sharp edges. The finer the grains, the sharper these edges can be-- with the most extreme being obsidian (volcanic glass), which can form from either basaltic or rhyolitic lavas.
Aphanitic (a = not, phaner = visible) rocks in contrast to phaneritic rocks, typically form from lava which crystallizes rapidly on or near the Earth's surface. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals. The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. An example of an aphanitic rock is basalt.