Black Onyx, Jet, and Black Glass were all used in the jewelry industry.  Although black onyx and jet were used long before the Victorian Period, the period had a huge impact on the use of black stones. At the time of the death of Queen Victoria's husband, Prince Albert, in 1861, mourning jewelry became the standard for 40 years.

Although it was jet that was mainly used for mourning jewelry in the 1800's, it is important to understand the different types of stones that are seen between the mid 1800's and the mid 1900's.

Let’s begin with understanding what each one is and their characteristics. Once you understand the characteristics it still is going to be a learning curve to recognize the differences but with a little practice, you will get the hang of it.

In the simplest of terms, Black Onyx is a stone.  It is considered to be a type of quartz.  Within the stone are layers of what is known as chalcedony – it is what actually makes up the stone.

• Hardness of 7
• Normally cut as a cabochon or into beads.
• Cooler to the touch than Jet
• Natural color is deep grey and it is heat treated to achieve the desired color
• If onyx is pure black, then it has been dyed to achieve the color
• Striped because of the layers of chalcedony

Jet is a product of high pressure decomposition of wood.  It basically comes in two different types; hard and soft.  Hard is a result of carbon compression in salt water while soft is the result of carbon compression in fresh water.

• Hardness of 2.4 although it will vary a bit
• Static Electricity is present when vigorously rubbed against wool or silk
• Can see growth rings under microscope (120x plus)
• Takes a high polish
• Will float in water
• Jet is easily damaged because of it relative softness
• Rub hard to warm the stone – slight smell of coal.  If it smells of rubber it is probably vulcanite and if it smells of formaldehyde, it indicates Bakelite.
• Always opaque
• Jet remains room temperature and feels warm to the touch
• Jet can be carved but it also easily damaged

Black glass needs little to explain it is exactly that; black glass.  Note that what is referred to a French Jet, is actually black glass.  In jewelry, it is referring to the color of the glass, not the type of stone.

• Hardness of 5.5 - 6.5
• Glass will sink in a glass of water
• Colder than Jet
• Is poured or molded
• Hard or high pitched ring
• Facets may be "soft"

As a note, one can do the hot pin test on any of the above but it is not advised.  You can damage the piece of jewelry or even destroy it, especially if the stone ends up being celluloid or plastic.  In fact a hot pin test on celluloid can be dangerous so it is not recommended.