This jar is made of black glass.  Generally, "black glass" refers to glass that is difficult to see through even when there is a light behind them although they usually aren't entirely opaque.  Some are deep purple such as the Hartell's, but this one is deep olive amber which is probably the most common case.  There is a small "+" sign between "Mason's" and Patent" on the front.  This jar is supposed to come with and without this "+" and the one with is supposed to be more desirable.  Like most black glass jars, this is an extremely rare specimen.  I doubt that 5 exist although I'm purely guessing at this time.  I think I've seen one other for sale in 25 years.

The black, like amber and cobalt, was designed to keep light out which would harm the contents over time.  There is actually a patent on this from the 1880s I believe and there was a flurry of experimentation back around that time.  The reverse of the jar can be seen here.  It's really the most important part since it is a version of the CFJCo monogram and marks the jar as being made at the Consolidated Fruit Jar Co.  The monogram doesn't look like most CFJCo monograms but is much blockier.  The reason is that it was adopted from another black glass jar with an HGCo monogram on back which stood for the "Hemingray Glass Company".  I have an example of that jar also.  I should get a picture of those two monograms together for comparison.