My priest uses a crystal decanter for celebrating Mass. Is this allowed?
If he is using it to hold unconsecrated wine that he then pours into chalices prior to the consecration, yes. However, decanters or flagons cannot be used to hold the precious blood. Neither can chalices made of crystal or pottery.
Redemptionis Sacramentum: Never to be used for containing the blood of the Lord are flagons, bowls, or other vessels that are not fully in accord with the established norms (106). Sacred vessels for containing the body and blood of the Lord must be made in strict conformity with the norms of tradition and of the liturgical books. . . . Reprobated, therefore, is any practice of using for the celebration of Mass common vessels, or others lacking in quality, or devoid of all artistic merit or that are mere containers, as also other vessels made from glass, earthenware, clay, or other materials that break easily. This norm is to be applied even as regards metals and other materials that easily rust or deteriorate (117).General Instruction of the Roman Missal: Sacred vessels are to be made from precious metal. If they are made from metal that rusts or from a metal less precious than gold, then ordinarily they should be gilded on the inside (GIRM 328).