Is Chanderi a silk or a cotton?

Traditionally, both. The classic Chanderi is woven with a silk warp and a cotton weft, which is where its particular half-transparency and faint sheen come from.

Chanderi is now also woven in pure silk and in pure cotton. Because the name describes a tradition and a place rather than a fibre, the fibre has to be stated separately — which is what the Fabric field on every product page is for.