silk floss; silk wadding
Refers to the soft, fibrous material obtained from silk cocoons, used for padding, stuffing, or as a material in crafts.
真綿でくるんだ宝石を箱にしまった。
I put the jewel wrapped in silk floss into the box.
真 (ま, 'true') + 綿 (わた, 'cotton' or 'wadding'), indicating genuine silk wadding as opposed to cotton wadding.