The Agate bowl (German: Achatschale) is a of the Habsburg family relic. The bowl originates from the 4th century, probably from Trier or Constantinople. It is 58 to 58.5 cm in diameter. The span across the handles is 76 cm. It is cut from a single block of hardstone and is the largest gemmoglyptic (carved-stone) bowl in the world.
From the 17th to the 20th century, there were descriptions of an inscription "B.XRISTO.RI.XXPP", which was seen as a reference to Jesus Christ. Since the writing seemed to be neither painted nor carved in the bowl, but to appear in the natural veining of the stone itself, the Agate bowl was regarded a relic. It was even considered to be the Holy Grail.
After the relic had an restoration in 1951, the inscription was reinterpreted by art historian Rudolf Egger to read "ARISTO", the name of the stone cutter. There are also concerns that the inscription is in fact an optical deception.
In 1564, the sons of Emperor Ferdinand I agreed that the Agate bowl, together with the Ainkhürn, a narwhale tusk considered to be the horn of a unicorn, should be an inalienable heirloom, owned not by one member of the family, but by the House of Habsburg as a whole.
Translated from the German Wikipedia entry. http://www.khm.at/homeE3.html