Holocrystalline describes igneous rock texture made of wholly or entirely crystals with no glass. Glass is an amorphous or uncrystallized solid or mineraloid.
Usually, rocks with this texture achieve 100% crystallization. Depending on the crystal size, they can be aphanitic, phaneritic, or medium-grained. Medium is in between aphanitic and phaneritic and occurs in rocks like diabase.
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Holocrystalline aphanitic texture describes fine-grained rocks with entirely small crystals that cannot be identified and distinguished by the naked eye. The typical grain size is less than 1/16 mm (0.0025 inches). To see individual crystals, you need a petrographic microscope.
Examples are mostly volcanic, but not all. They include andesite, rhyolite, basalt, and dacite. These rocks form from the fast cooling of magma on or near the Earth’s surface.
However, most of these aphanitic rocks will have a mixture of crystals and glass. Such will no longer be holocrystalline. Instead, we call them hypocrystalline.
On the other hand, holocrystalline phaneritic rocks have entirely large, visible crystals. You can see these and distinguish these crystals with an unaided eye.
Examples are plutonic rocks like gabbro, granite, diorite, granodiorite, monzonite, tonalite, norite, etc. They form from the slow cooling of magma inside the Earth’s crust. The slow cooling favors large crystal growth.
Lastly, holocrystalline rocks may also show other textures, including porphyritic, pegmatitic, etc. Also, they can have nearly equal grain sizes, i.e., equigranular, etc.