A synthetic fibre with a lot of greenwashing.
Generally these are made of a few layers (2-3 maybe) of standard microfibre, wrapped in an outer grey fleece. Because the microfibre layers are covered in fleece these inserts can go directly against your baby’s skin and will provide a stay-dry layer.
The outer fleece is reportedly made from bamboo nanoparticles with are heated to an extremely high temperature to burn them. Hence charcoal bamboo. Although I do have reservations as to whether the bamboo nanoparticles bit is true in every case.
Manufacturers can get away with calling them ‘charcoal bamboo’ because that is a correct (albeit slightly misleading) description of the outer fleece. But this fleece does not have any of the properties you might typically associate with bamboo, such as good absorbency, because it’s a fleece (fleece cannot hold liquid).
Often marketed as a bamboo insert with odour reducing properties (which isn’t actually true). Their only real benefit is that they can hide stains, but then if you’re having persistent stains you need to troubleshoot your wash routine, not hide them.
In reality these inserts are often just microfibre. Of course microfibre has it’s place, but if you’re purchasing charcoal bamboo and expecting bamboo qualities then you may be disappointed.
There are of course actual charcoal bamboo inserts available. These have the same charcoal bamboo fleece (remember it’s just a grey fleece, there’s still no odour reducing properties), but inside the insert are layers of bamboo not microfibre. These will perform in the same way as a normal bamboo insert. It benefits from having a soft stay-dry layer but this also adds an extra bit of bulk so consider whether you need inserts that are able to go directly against the skin.
If the manufacturer or retailer doesn’t explicitly state the content of bamboo under the fleece, then assume these are microfibre.