Microcrystalline means that the crystals are not recognizable with the naked eye or with a standard microscope.
Minerals made up from even smaller crystals are said to be cryptocrystalline from the Greek word 'Kruptos' meaning hidden and are classed as being massive in their habit because they have no orderly structure. Most are semi-transparent to opaque.
With modern day microscopes with high magnification power, these terms don't seem valid anymore and are referred to as polycrystalline.
Includes stones classified as varieties of the quartz species: