The product name includes the range 3–7 cm, which indicates the size of sphere the stand is intended to hold. In typical use, the ball sits in the recess cut into the top of the stand, so the most reliable fit is a shungite or decorative ball whose diameter falls inside that range.
To check compatibility, measure the ball at its widest point. A ball near the lower end of the range will sit lower in the recess; a ball near the upper end will sit higher. Either can work, but the most stable match is a ball that rests in the recess without touching the surface around the stand.
If the ball is noticeably smaller than 3 cm, it may wobble; if it is larger than 7 cm, it may not seat properly and could be top-heavy. If you are choosing a ball separately, keep the diameter within 3–7 cm and remember that the stand itself is small, so it suits a compact display rather than a large sphere.
Because the recess is shaped for a sphere, the stand can also hold a decorative stone ball of the same diameter, not only a shungite ball. The material of the ball matters less than the size.