well i was pointed to this post and asked to comment so here goes the fist image is what appears to be a dolomite with a great many vugs giving is fragility. this rock i very common especially in the Mohawk valley of NY were the Herkimer diamonds occur.Thanks Ian, looks like you nailed it;
http://www.adaeuro.com/catalog/English/09 Layout Materials.pdf
what is odd about Ohko and all the stones sold by ADA is when you google them, only ADA relevant links come up, which leads me to believe they basically made up names for all the stones they advocate and resell, and that in actuality they most probably called something very different by true geologists.
I've no idea, but there's also the possiblity that if you knew the Japanese character for "manten" that it's a common name for this type of stone in Japan...I know this is an old thread, but the banned spypet brought up a good point, and it's something I've been recently coming to terms with myself... these rock names are made up!! See for yourself, everytime you google a rock name, it only returns ADA results for the most part.
As sad as that is, I believe someone like metageologist here could probably reveal the REAL names of these rocks, and finally the average person such as myself can afford a nice hardscape!
I want to know what the REAL name of manten stone is.