This is a beautifully patterned slab of Crazy Lace Agate. There are lots of fine fortification lines in whites, reds, and browns. There is also a nice drusy pocket on one side which can make for a stunning looking cabochon.

It is about 5.5 mm thick.

Photographed wet to show details and dry to show the slab dimensions(second photo).  The dry photo is on a 10mm or 1cm to the square scale. Both sides of the slab are shown wet because the lace pattern varies so much.