Gemstones misnomer are misleading names for gems.
Perhaps these names are given to romanticize and glorify their names so that a seller would be able to sell the gemstones easier. Some names may originate from their locality, in part for want of a better name initially when they were first discovered. Or these names are meant to mislead, as they almost resemble the original mineral material.
There are many gems misnomer for diamonds. The most common one is called Herkimer Diamond, which is actually clear rocky quartz. Some clear quartz are also termed as Alaska Diamond or Arizona Diamond.
In China, all green stones are termed as Jade (as in the Chinese character of Jade, 玉). So one has to be careful of what the seller represents when buying such green gemstones or carvings.
Misnomers are different from trade names.
For example, to the Chinese, Imperial Jade is the trade name for Burmese Jadeite Jade. Jadeite Jade is also known as Hard Jade, whereas Nephrite Jade is known by its trade name of Soft Jade.
A picture of the common gemstones misnomer is presented so that the reader can easily associate such misnomers to their original mineral.
Just browse through it and you will remember it, in case you may come across such terms in future.