This princess star cut faceted gem will fit any Industrial Strength brand barbell of the same gauge. The 18g/16g gem end will fit either gauge, and same for the 14g/12g, and the 10g/8g. The gem size is 6mm.
This threaded end will fit Industrial Strength brand internally threaded jewelry, but may not fit your other brand jewelry if purchased elsewhere (due to possible differences in threading). The gauge of the end must be the same gauge as the jewelry it threads into.