NT931
1872
Titanium, Tantalum and Platinum
Because they rhyme! 
I enjoy the lightness of titanium, especially on this "Sakura" GS. Such a joy on the wrist, and Seiko manages to polish Ti in a way that makes it glow.

Tantalum, for that distinctive blue-ish grey hue that complements the dial of the CB.

And platinum, for the heft, and the sheer pleasure of its weight!