Fabric swatch two and a half inches by four inches. White brocade with silver thread embroidery. Label in box says: "piece of the wedding dress of Madam Washington - Presented by Mrs. Lindsley, of Nashville, Tennessee. Label on back of box says: "Piece of Mrs. Geo. Washington's wedding dress By Mrs Lindsley".
Probably part of the William Howard, Sr. collection.
About the dress:
"Martha was married in a yellow brocade dress that was trimmed with lace at the neck and sleeves. Some historians describe her wedding dress as a gold damask dress.
Underneath her gown she wore a white silk petticoat with silver threads. Her shoes were purple satin and trimmed in silver metallic lace and sequins. She wore pearls in her hair.
George wore "a blue suit with a white satin waistcoast and blue buckles on his shoes.""
Source: Ruth Ashby, George & Martha Washington, page 17.
It seems likely that our swatch came from the petticoat.
As to this being a genuine piece of her dress, this discussion from the Washington Post on February 2, 2009:
Brigid Schulte: The curators at Mt. Vernon said that many of Martha's descendants decided to cut up her clothes so that each could have a piece of her, so to speak. Along with the shoes is an oddly shaped piece of cloth, believed to be part of her gold damask wedding dress.