geology
Hi, Phil–
Welcome to the forum/section!
My guess for natural causes of blue in agate would be that traces of copper
compounds are present. Chrysocolla (copper silicate) is a copper mineral that
could easily be mistaken for quartz (of which agate is a form), and is found
mixed into quartz at times. Many other copper minerals are blue, such as
turquoise and azurite; it is a characteristic of the copper ion.
But my best guess, unless she found it or knows that it was blue when found, is
that it has been dyed blue. A lot of agates sold by dealers are dyed, and blue
is a less common natural color.
–Doug