The ornamental design for a precious stonecut of a bottom portion with eight chevrons, as shown and described.
FIG. 1 is a top plan view of a precious stone cut of a bottom portion with eight chevrons showing my new design;
FIG. 2 is a side view; and,
FIG. 3 is a bottom view.
The broken line showing of the crown of a gemstone is included for the purpose of illustrating environmental matter and forms no part of the claimed design.