This is a Japanese ojime, used as a slide to hold together Inro case sections threaded on a cord running through extreme ends of each case section.

This one I believe is bone, possibly early 18th century.

In Japan wealthy persons devoted much attention to assembling philosophically and spiritually appropriate combinations of decorated Inro, ojime and netsuke (the toggle at the end of the cord, used to secure the case on a belt or sash). The combinations can have extremely subtle or esoteric meaning. The Inros tended to have the shortest life integrity, so often netsukes and ojime became separated from the assembly and many are now sold separately. In addition when an Inro changed hands, the new owner often wished to change the combination according to his own taste and wherewithal.

Considerable artistic skill and other skills were directed to making the inro, ojime and netsuke.

Ojime may be made of stone, bone, ivory, wood, copal or amber, or a range and/or combination of metals available to the Japanese artisan. They may be carved, lacquered , inlaid, etc.

The perforations of many ojime are lined with a metal sleeve, held in place by the sort of decorative collar shown in my example. This was meant to minimize abrasion of the cord by the perforation edge.

This one varies in diameter a little, given the varying depth of carving. Its size can be estimated by the millimeter scale shown with the picture of the perforation.

Regards,
Jadeterrace