This is a cornflower blue Pacific Glassworks half gallon jar.  As you can see, it has "San Francisco" embossed between "Pacific" and "Glassworks".  Most jars were made either on the east coast or midwest.  Jars from the west coast are rare in comparison and certain collectors, especially those who live on the west coast, seek them out avidly.  Since this jar was made in San Francisco, it falls into that category and accounts for a lot of its appeal.

It's essentially a modified Victory jar as can be seen from the reverse embossing here.