What can be considered a veil to be used in a bedeken at a wedding?
(See also my other related question)
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Nit'ei Gavri'el on the topic quotes the Shulchan Aruch to say that the veil should be opaque from the outside and should not be ornamented with gold and silver thread. The reason for opacity seems to be to demonstrate that the marriage is unconditional on factors such as money because if the bride cannot see what the groom is holding as he approaches her she cannot possibly be making her decision to marry him on the basis of its value. The rather extensive footnotes point to but do not explicate the reasoning for the limitation on gold and silver thread.
In more extensive footnotes he also cites practices of several sources to make the veil out of a special preexisting object - cloth used to cover a shofar, material used in performance of some other mitzva or even a previously-owned piece of fabric that had belonged to a tzadik.
Anything that covers the face is fine. There are those that use thick material which is not see through, and there are those that use material that is very sheer.