This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
Bamboo is not a wood but a grass. Several thousand different species
exist, ranging from tiny plants to huge towering trees. Many species
are light yellow, tan, or green; some can be purplish or black. Most
species are solid in color; some can be striped or spotted.
Bamboo stalks are hollow and segmented with solid portions of culm.
Bamboo is lightweight; its cross-section may be round, oval, or
slightly cardioid (heart-shaped). The outside of the bamboo is
naturally smooth and protective and should not be removed to make
plugs. The inside is normally whitish and may have a papery lining
which is usually removed or is shed over time.