Following on from previous post, this issue's topic covers the Saya (sheath). Saya are typically made from Honoki (magnolia wood) and lacquered. Honoki wood is a soft wood that is low in oils and hence material of choice for saya making.
The specific curvature (sori) of an iaito blade meant each saya is closely designed to fit that particular blade. This also implies if an iaito's sori has been altered for whatsoever reasons (rare instances), a re-design of the saya is also needed. Hence, the close relationship between blade and saya.
Fun fact: One example of an old saying that exemplifies this is "Moto no Saya ni Osamaru" - returning a blade to its original saya, used to describe reconciliation between two people following an argument.
Lacquering of the saya can be traced to as early as the Nara period. Originally meant to protect the saya from the elements, more advanced lacquering techniques gradually emerged with evolving aesthetic demand and creativity of the craftsmen.
Besides japanese lacquer (urushi), other materials used in saya lacquering can include tofu, egg white, powered stone, seeds or leaves such as rapeseed/ palm fiber, charcoal, eggshells, ray skin, shark skin, mother-of-pearl, precious metals and powdered metal. Variations in lacquer applications gave rise to a spectrum of matt and gloss finishes. There are also examples of more flamboyant saya finishes with 1) motifs such as family crest (mon) or mytical beasts in gold or 2) designs with auspicious meaning such as akebono (daybreak) and hiwari (cracking of ice underfoot as one steps out to enjoy sunrise on the first day of the new year).
Left: Saya designs (#1 to 5; top to bottom):
#1,3,4.Azuki matt, #2.Gold cha gloss, #3.Kuro matt.
Right: Closer look at #2.
 Source: "Swords of Japan", Yasuko Kubo, Paul Martin.