Is correct, karasu is a pattern that occurs in many different mines.
Traditional karasu usually occur in the lower, harder layers, aisa, namito, etc - and these karasu layers can be present (or not) in many mines. Because they are from lower layers, they are usually, but not always, harder and finer than upper layers. But this is a property of being from a lower layer, the pattern has nothing to do with the hardness or fineness.
The upper suita formations also would show karasu patterns sometimes, but these are not usually as hard or fine as the lower layers.