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 increase the visibility in dark levels, unlike 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. They will also show any other hidden items inside blocks.
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.
However, unlike the Udjat Eye, they will not help you find the entrance to the Black Market.
Instead, the unique function of the spectacles is the extra visual acuity provided in dark levels, which helps the Spelunker see a little further into the darkness, although holding the torch with the Spectacles will not further increase your vision range.
This is a very useful feature, as dark levels are considered to be one of the most dangerous environments in the game. The extra visibility can be the difference between life and death.