I have picked up a few chips of this intriguing agate over the years, mostly at estate sales and gem & mineral shows. If you do a google on 'blue lace agate' you get pages of the metaphysical properties of the stuff, so it must be good, right? I suspect it comes from different locations around the world and some has deeper blue coloring than others.
Since agate is common among the semi-precious stone family, I suspect the blue coloring comes from a particular mineral being present as silicified water settled into the seam in which it was formed over millions of years.
The density of this stone, along with ethereal coloration makes this a desirable stone for jewelry.