What spiritual or symbolic meaning is in agate?
The Holy Grail was carved out of a single piece of it.
"In fact, the Cup is made of agate stone - a popular material for drink vessels in those times. It is a homogenous piece cut out entirely from a large (sic) chunk of agate, 9 cm in diameter. Naturally, decorations of gold and pearls were added to the supporting structure over the centuries."
http://www.valenciavalencia.com/sights-guide/holy-grail-valencia.htm
image:
http://tejiendoelmundo.files.wordpress.com/2009/01/400px-santocaliz.jpg