Once the Rabbi who presides the wedding (Mesadder Kiddushin) pronounces the first two blessings (see here), the ceremony of Kiddushin (=consecration) takes place. The consecration of the bride is done by the means of a qinyan (a legal process) by which the bride and the groom are officially engaged.
This qinyan could be done in a different ways. For example, by the groom giving the bride a document (shetar) that states his intention to betroth her in accordance to Jewish Law. Alternatively, it could be done by the groom giving to the bride anything of a minimal value, not necessarily a ring. However, the custom in our days is that the qinyan Kiddushin is performed by giving a ring--the wedding band--to the bride.
There are different traditions regarding the metal used for the band. In some communities they would require to use only a band made of gold, or silver, etc. In our community we allow any metal. The ring, however, should be plain without any stones or accessories. This is why grooms would refrain from having any inscriptions on the back of the ring, so its value is clear and visible.
If the ring is lost and could not be found, technically, any other ring or anything of a minimal value could be used by the groom to formalize the act of Kiddushin. Provided the object by which the Kiddushin is performed belongs formally to the groom. This is the reason the Rabbi would make sure that the ring belongs to the groom and, for example, he has not yet given it to the bride, etc.
It is only the groom that gives a ring to the bride. In our community we ask the groom to wear his own ring (if he wishes to do so) before the Chupa begins.
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From Times of Israel