While I think this is something that must be appreciated in person, the panel accompanying the replica provides a good description and gives its background:
To see this Magna Carta replica, you will need to be on an official tour of the Capitol. Lucky for you, the Library of Congress Jefferson Building is connected by a tunnel to the Capitol Visitors Center, so you can pop over after seeing our Magna Carta exhibition.
Magna Carta Replica and Presentation Case
This display features a presentation case with a gold replica of the English document whose principles underlie much of the United States Constitution. Magna Carta (Latin for “Great Charter”) was sealed by King John of England in 1215 after his barons, dissatisfied with his capricious rule, united to limit the king’s powers and forced him to agree to its contents. Magna Carta forbade arbitrary arrest and imprisonment, established rights to a fair trial and to security of property, and guaranteed that the nation’s government was bound by the same laws as its subjects.
Centuries later, the English colonists in North America were inspired to reassert these rights and liberties. America’s Declaration of Independence and Constitution were founded upon the basic principles of Magna Carta.
The stainless steel presentation case, which is clad in gold and white enamel tiles intended to suggest feathers, contains two gold panels.
The panel in the front is inscribed with a replica of Magna Carta and holds gilded replicas of the two sides of King John’s seal. On the vertical glass panel is the English translation of Magna Carta.
On the gold plate at the bottom of the upper half of the case, images of opposites, including the sun and the moon and Adam and Eve, are engraved in contemporary stile and highlighted with pearls and gems. Fifty diamonds above a dove represent the fifty states. Above the plate a three-dimensional tree of life springs from the four rivers of paradise. Around the trunk a snake, representing evil, coils below the Apples of Original Sin and mistletoe. The tree branches hold the Tudor Rose for England, the shamrock for Ireland, thistles for Scotland, and daffodils for Wales. From oak leaves and acorns, for Britain, rises the Royal Coat of Arms, with the gold lion and silver unicorn set with precious gems under a crown. The brilliant colors were created with enamel.
The presentation case rests upon a slab of polished pegmatite, a volcanic stone. The pedestal of the display case is made of Yorkshire sandstone.
The presentation case was designed by Louis Osman (1914-1996), who had made the crown for the investiture of Prince Charles. He was assisted by thirty craftsmen, among them his wife, Dilys, who did the enameling. The display was presented to the United States Congress by the Parliament of the United Kingdom to celebrate the bicentennial of American independence. Representatives of the Parliament and the Congress formalized the gift at a ceremony in the Capitol Rotunda on June 3, 1976. The earliest extant original copy of Magna Carta was displayed in the case atop the gold replica for one year and then was returned to England.
The display remained in the Rotunda until 2010. During its move to this location, the presentation case was conserved.
The Library of Congress is commemorating the 800th anniversary of Magna Carta with an exhibition – Magna Carta: Muse and Mentor, a symposium, and a series of talks. Through January 19, 2015, the Lincoln Cathedral Magna Carta, one of four remaining originals from 1215 is on display along with other rare materials from the Library’s rich collections to tell the story of 800 years of its influence on the history of political liberty.