(above) osmium atoms arranged in a hexagonal close packed pattern
Osmium: metal whose atoms are arranged in closely packed hexagonal layers. First and third layers are in line with each other. This gives what is known as a hexagonal close packing. However, in osmium, the Os-Os bonds are of two different lengths, making the hcp unit cell relatively shorter than an ideal hcp. Consequently, the osmium atoms are drawn as ovals. This flattening top to bottom is just discernible in this graphic.
Image is 500 pixels across, the originals are 4096 pixels across.