MARINWOOD, Calif. (AP) _ Jewelers say it's plain old tarnish, but at least 18 Roman Catholics say it's a miracle: They report that their rosaries have changed from silver to gold or copper.

The believers dismiss the scientific explanation of the phenomenon, instead crediting the Virgin Mary for the yellowish color of their formerly silver rosaries.

''It was like Mary came to me,'' said Lila Somers of Novato, clutching her rosary in a church's Bible study room where she went this week to hear other tales of rosary miracles.

''The rosary was in my hand and it was glowing,'' she said.

Somers and 17 others have reported the color change since a retreat in Burlingame last month that focused on Mary. About 50 members of the Mother of God Queen of Peace Prayer Community of the Carmelite Monastery in Marinwood, encompassing two prayer groups, attended.

They say the experience has changed their lives and encouraged them to pursue their faith in a deeper way.

''Chills went through my whole body,'' said Mary Senour of Petaluma, who said she saw her rosary turn from silver to gold as she drove home from the retreat 15 miles south of San Francisco. ''I just felt humbled, like God was really blessing us.''

Jewelers say the erosion or tarnishing of the silveer coating is influenced by body moisture and environmental factors such as humidity and climate.

''The one that we inspected is made out of brass and white base metal, and under certain conditions base metals will oxidize and corrode,'' said Evan Hacker, a Mill Valley jeweler who examined one of the rosaries. ''It's an oxidation process better known as tarnishing.''

Miraculous changes in the prayer beads have been reported in other parts of the world.

In Portsmouth, R.I., at least 50 parishioners reported their rosaries had changed from silver to gold after a man returned last June from a pilgrimage to Medjugorje, Yugoslavia. Children in that community claimed to have seen the Virgin Mary many times.

Mary Ann Serrano, a San Rafael evangelist, has visited Medjugorje and said there are numerous other associated phenomena, including a crackling fireball and the sun spinning across the sky.

''Maybe we're a little too enthusiastic,'' she said of the local incident. ''But it's real. It's the realest of the real.''

The Rev. John McGregor, a part-time chaplain for the Carmelite sisters, also dismisses the scientific explanation.

''I don't really think it's a chemical reaction from their sweaty hands ...,'' he said. ''I think the phenomenon is much like Moses and the burning bush. It calls people's attention to prayer.''