One of the most reproduced fruit jars are the 1858s.  Beginning in the early 1970s serious attempts were made in this country to forge these jars and sell them.  The first batch I remember  were 1/2 gallons supposedly made in Kentucky or Tennessee.  Most of the long time collectors noticed something wrong with the color almost immediately.  The lips were ground but obviously new.  Many of these had the mold number 851 on the base. They were made in many colors, black, emerald green, cobalt blue and olive green.  Not all are base marked with the 851 number. The colors are unusually bright and the glass has a different sheen to it than originals.  Despite these being modern they still command prices between $40-$100 depending upon the color. the milkglass example shown below just sold at a local show for $40.

Other reproductions of the 1/2 gallon may have the mold numbers 1171.  They can be found in Milkglass, amber, lime green cobalt and aqua. These jars have unlettered Hero cross on the reverse. Mason's CFJCo Patent Nov. 30th 1858 jars were also reproduced in 1/2 gallon size in black purple, emerald green, amber, and cobalt.  The bases are unmarked. Lesson:  If you are buying a colored 1/2 gallon- know who you are dealing with especially if you are not knowledgeable yourself. Originals will bring many times the price of the reproductions. If it looks too good to be true it probably is.

milkglass 1858 Mason jar
Repro milkglass half gallon

Midgets, popular with collectors were also reproduced.  Some more convincingly than others.  The most convincing ones have the mold number 971 on the base. They can be found in milkglass, amber, emerald green, cobalt blue amber and even carnival glass. Less convincing reproductions like the three below are easy for even the novice collector to spot because of the odd colors and new lids..

Left is an Amber midget base marked 971.  

Watch out!! The unscrupulous often find old caps to put on new jars. 
They might be scuff the bases to mimic wear.  

Within the last few weeks a new batch of reproduced 1858 jars  imported from China have shown up.  These jars lack the ground lip of earlier original 1858s and come without caps. there are three sizes, pint, quart and the half gallon.  They are realistic with respect to the embossing but not good color reproductions. They apparently were blown in six different colors as shown below: cobalt, purple, emerald green, light amber, ruby and yellow green.  John Bartley, dealer and organizer of the Springfield bottle show, (shown below) brought these jars to collectors attention this last week.  Please note-John is NOT the person importing them.  He happened upon some of the first batch brought in to this country.

Base embossing of the half gallon

Cobalt blue quart

Pint Base Embossing

Watch for these new bottles to make an appearance in an antique mall near you.