In the formation of an ordinary agate, it is probable that waters containing silica in solution percolated through the rock and deposited a siliceous coating within the vapor-vesicles. Variations in the composition of the solution or the conditions of deposit may cause corresponding variation in the successive layers. As a result, bands of chalcedony often alternate with layers of crystalline quartz. 
3 x 2 x 1
wt. 0400 lbs