Identifying valuable Mason jars can be difficult because there have been several replicas made over the years and because authentic Mason jars are still being produced under the names Kerr and Ball. Invented and patented by John Mason in 1858, Mason jars were one of the first glass jars to be used for home canning. Vintage Mason jars, if they can be found, are rare and worth a lot of money.
Look at the colour of the jar. Originally, Mason jars were an aquamarine colour. These are the very first Mason jars ever created, and they're quite valuable. Later, pink chemicals were added to the glass in an attempt to make it clear. Though this did nothing more than turn the glass a murky, purplish pink, these Mason jars have been appraised at £27,300.
From 1900 to 1920, Mason jars were created in many colours, the most popular being amber, burgundy, and cobalt.
Examine the bottom of the jar. Richard Cole, a Mason jar aficionado, says that Mason jars were embossed with the words "Mason's Patent Nov. 30, 1858" up until 1900, 21 years after the original patent expired. Newer jars don't have this stamp.
Look at the jar's lid. The original Mason jar had a rubberised seal on the centre of the cap that allowed the lid to stick to the jar with suction, preserving the food inside. This is the design that John Mason patented. If a Mason jar doesn't include a rubberised sealing lid, it's worth significantly less.
Find square Mason jars. They have an embossed Ball label in the centre of the jar, and they also have the patent stamp on the bottom of the jar. These jars were originally used for holding moonshine during Prohibition. They are no longer made and are extremely rare.
If you think you've found a collectable Mason jar, take it to an antiques dealer to have it appraised.
If you find old Mason jars with food in them, throw the food out. It's definitely not safe to eat.