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!