This 4.21 shaku White Oak Jo 1-sun (approx 30.3mm) is both a durable and heavyweight Jo for advanced practitioners and hard contact practice. Please see the pic 2 for diameter comparison (first from right).
What is Japanese White Oak?
The White Oak Wood used for Japanese Bokken, also called 'Shirogashi', greatly differs from the White Oak Wood referred to when speaking about North American Bokken.
While the North American Oak and Japanese Oak are related, the Japanese White Oak tree is evergreen, due to this it does not have a conspicuous open grain like the American White and Red Oak.
The open grain structure, of the American Oak wood produces soft areas which are more prone to impact damage.
Shirogashi on the other hand has a structure that is uniformly hard, has excellent dent resistance and has better impact strength than American Oaks.
Compared to Akagashi the Shirogashi has denser grains which also makes it slightly heavier and more preferable as material for Bokken.
Because of the durability and weight of Shirogashi, it's widely regarded as great material for making high quality Bokken and Jo.