We explained previously that the general custom today in most Jewish communities is to use a ring for the act of Kiddushin (see here), which will be giving to the bride by the groom. The general custom is to use a gold ring, since gold is a noble metal, and according to some Rabbis, because gold is a metal associated with the Gan Eden (see Bereshit 2:11), where the first couple lived harmoniously.
In some communities they use a silver ring (Ben Ish Chay), however, Bet Yosef mentions that a silver ring should not be used (Eben haEzer 30). in any case it is important that the true metal out of which the ring is made not be disguised (gold plated, etc). The true value of the ring should be perfectly obvious because the qinyan kiddushin (=the legal procedure of the marriage) is not done with the ring itself but with the monetary value of the ring, and the bride should be able to ascertain the value of the ring at a glance.
For the same reason, it is a long established custom not to to use a ring that has any kind of stone in it. A stone is a blind item whose value is not readily apparent. Some Rabbis explain that the reason--perhaps, an additional reason, which was mentioned by Keter Shem Tob--for not using a ring with a stone, was so as not to embarrass a poor groom which might not be able to afford a ring with a stone. So every Jewish groom uses a ring which is affordable for everyone else.
It is also customary to use a simple ring without any design, inscriptions or engravings. An engraving or design can modify the ring value in a way that is not easily ascertainable. In our community although we tell the groom to bring simple ring if involuntarily they bring an engraved ring we still use it normally.
(Adapted from "Made in Heaven", by Rabbi Aryeh Kaplan)