Interesting, informal, safe bet... that must be
a rope 
Although, it wouldn't hurt if the said long strand was multicolor. I just couldn't find the right example. Folks mix silvery Tahitian pearls, pink and peach freshwaters, cream and golden pearls quite often now. Strands come with mixed and matched colors and shapes too. Delightful!
Classics are fine. Just keep in mind that some are rightfully labeled 'typical-male-presents'.
THIS is a tame example, and still...
One of those 'true blue' pearls could be the focus there. Three or five throughout a choker of silver gray sounds truly fun.
It is so easy to dream up pearl choices!