Spectacles are an accessory that improves the Spelunker's vision. They can be bought from certain shops, occasionally found in crates or received as a gift from Kali after making enough sacrifices at one of her altars.
Spectacles are rarely considered to be worth buying from shops due to the functional overlap with the Udjat Eye, which is gained for free and is guaranteed to appear.
They are not completely worthless however, as they do indeed have a useful advantage over the Udjat Eye.
Spectacles function similarly to the Udjat Eye. Like the Eye, they show you hidden treasure deposits inside blocks that are not visible with the naked eye (such as jewels and gold nuggets), which increases the productivity of mining with bombs or a mattock.
While they will not help you find the entrance to the Black Market like the Udjat Eye does, they will instead show you hidden items inside blocks, which the Udjat Eye cannot do.
Not only does this include the guaranteed items such as the Mattock hidden at the bottom of a Snake pit, but also the items that rarely generate randomly inside of any solid block, including (but not limited to) accessories and weapons.
The spectacles are the only way you will be able to see these items. Without them, it is unlikely you will find these items by chance, so acquiring spectacles can often lead to gaining more useful items for free, as long as you have the means to excavate them.