1. cryptocrystalline - Noun
2. cryptocrystalline - Adjective
Indistinctly crystalline; -- applied to rocks and minerals, whose state of aggregation is so fine that no distinct particles are visible, even under the microscope.
Source: Webster's dictionaryNevertheless, opals can be roughly divided into those that show no signs of crystalline order ( amorphous opal) and those that show signs of the beginning of crystalline order, commonly termed cryptocrystalline or microcrystalline opal. Source: Internet
This is thought to be because chalcedony is extremely finely grained (cryptocrystalline), and so has a very high surface area to volume ratio. Source: Internet
The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Source: Internet