Silk is a natural protein fiber, some forms of which can be woven into textiles. The protein fiber of silk is composed mainly of fibroin and is produced by certain insect larvae to form cocoons.
According to Ayatullah al-Khu`i: Wearing a dress made of 100% silk is haram for men at all times.
However, he says: "There is no problem in carrying it [an item made from 100% silk] in salat and outside salat or in using it as bed-sheet or mantle --- as long as these items are not identified as `dress'.
"And there is no problem in making the hem of dress with it [100% silk] although it is ihtiyat wajib that the hem should not be more than four fingers in width.
"And there is similarly no problem in using it [100% silk] for waist-band...lace/braid...even the inner-linning of suits, etc.
"But a dress (libas) which is not sufficient [by itself for appropriate covering] in salat, then it is ihtiyat wajib to refrain from it." (Minhaju 's-Salihiyn, vol. 1, p. 139)
& ** In both these cases, Ayatullah Sistani says it is ihtiyat mustahab, not wajib. (Minhaju 's-Salihiyn, vol. 1, p.179)
In the light of the above, the use of a dress with pocket square would be allowed. About the neck-tie, I am still not sure how we would classify it -- as an item of dress or a decorative part of it?