They look like mini books, these Sailor Shikiori ink cartridges. The booklets are small boxes folded from beautiful paper and have an elegant print. At the back of the booklet is a tab with which you open the box. It contains a small case with three colored ink cartridges.
The Sailor Shikiori ink cartridges come in all twenty Shikiori colors. Ideal for the Prof Gear Slim Mini that doesn't accept a converter (!) and practical for all other Sailor fountain pens. They are not cheap but yet sublime!