This black volcanic sand comes from Japan. It is suitable to mix it with the soil or to be put in the saucer of your bonsai as an embellishment. The slightest irregularities (like the wobble) are thus compensated.
The porous rock absorbs a lot of water which then evaporates. In this way a higher humidity level for your bonsai occurs.
The Fuji sand is a kind of volcanic gravel similar to the Kanuma soil and Kiryu sand. The name comes from the production done near the Fuji mountain.
The Fuji sand has the best drainage and ventilation among the volcanic gravels.
As the Kiryu sand, it's a soil that is often used to plant bonsai and wild grass that don't love over-humidity.
The Fuji sand is used as weel as decorative sand to adorn the surface of the pot.