Through hardened and ground. Polished.

Collets ER16, ER25 and ER32 without a seal also available in inch sizes via the eShop.
The collet clamping ⌀ range is always up to 1 mm less than nominal, e.g. ⌀ 10 − 9 mm, except for ⌀ 1, 1.5 and 2 (where it is only 0.5 mm).