Sagenitic agate is any agate having acicular or needle-like mineral growths inside.  These hair-like filaments are often arranged in fans or starbursts.  The inclusions come in a wide array of colors.  Sagenite has been found in over 250 different agate deposits worldwide, a little in most agate fields, probably less that 5% of the available agate in most fields.  This agate is from California and contains mostly yellow sagenite but also a few plumes; can you spot them?

Source: The Gemrock, 7/2014

Update: It looks like somebody at IGAMS (and the CMS Tumbler, The Clackamette Gem, and The Glacial Drifter, all of whom were listed as sources) might be a fan of Pat McMahan’s website Agates With Inclusions, as the text from their article on agates that I read is very similar to this article on sagenite and plume on his website.  So, thank you for the inspiration, Mr. McMahan!