Here is a portion of a gold crystal. The gold lattice is ccp (cubic closest packing) or fcc (face-centered cubic). The ccp stacking arrangement is ABCABC... and here the layers are (mostly) shown as blue (A), green (B) and red (C). Several features have been colored differently to call your attention to them. The four atoms that are orange (and partially transparent) form a tetrahedral hole, while the six yellow (and also partially transparent) atoms form an octahedral hole.