I went to Dorozome park in Amami.
It is far 15min to use a car from where i live.
In the park there are many marsh.
A soil which in a marsh is red clay, like a power and has a lot of iron.
Iron in the soil makes fixing color in textile.
The iron content is the highest place this island than.
It will make a pure black in silk.
So its called "Black diamond".
photo by http://amami7.seesaa.net/article/119735412.html