If this piece of rock was taken from a stratum of similar composition-- "all black sand with cement"-- then it is unlikely a positive sign as the majority of readers will attest to. However, if it was a discrete fragment imbedded in an "alien" ground, then it's possibly a sign. Again, Rangler, Casca and others have aptly mentioned that a sign to be a valid one has to be taken in the CONTEXT of the environment where it was found. It has no value by itself.
Even if one has a book of Japanese treasure signs, knowledge and familiarity will not be of much use unless a deeper appreciation and understanding of what they represent exist.
At this point I would just like like to remind myself as well as others that the ART of deception (i.e., use of decoys, etc.) is totally different from the SCIENCE of concealment with regards to the burial or hiding of looted treasure (Yamashita gold!) as done by the Japanese during World War II particularly in the Philippines.
If one is fortunate enough to see through the "haze and maze" as deception and concealment are intertwined, then it is possible to find the safest and shortest distance from point A (portal of entry) to point B (treasure location) as I've always believed that the people of the JIA had a shortcut for every hidden wartime loot and more importantly one that is free from hazardous obstacles like booby traps.
--tabu
"To know is good. To understand is better. To realize is best."