Obsidian is a pure natural glass of ryolitic composition, black to gray in coler, naturally vitreous in appearance, and usually exhibiting a striking conchoidal fracture. Obsidian may be easily trimmed up to any desired shape --- to a point or sharp cutting edge, for example ; therefore this rock was largely used by the ancients in a manufacture of stone implements and weapons.
Rock fragment of obsidian occur associated with the other vestiges from the histric relics of Jomon age in Shimane prefecture.
The obvious questions are how and from what is the obsidian fragment derived. It is a irnportant task to clarify the origin of these obsidian fragment for studies of econornical interchange and cultural aspect at that time.
In order to solve the question on the origin of obsidian fragment from the historic relics of Jomon age, the samples of obsidian were taken from nine districts in northern Kyushu and two districts at Oki island in Shimane prefecture, obsidian as raw material presumably were derived largely from these districts.
Detailed studies was carried out on these samples, by means of chemical analysis of groundmass of obsidian by EPMA. And these analytical datas were compared with those of obsidian fragments from various histric relics of Jomon age on major element chemistry. The results leads to the following conclusions that characteristic difference found in each samples and their origin can be distinguished by their chemical composrtion of groundmass.