I first read about them as a teenager and was captivated by their variety, delightful decoration and dainty tiny-ness.
Dating from the 1720s these tiny cases generally contained items used daily such as tiny folding scissors, needles, thimbles, bodkins, tweezers, nail files, little folding knives, spoons, hairpins, makeup pencils and even tiny vials of perfume.
They were most often carried in pockets, although I believe some were suspended from chatelaines - another delightful accessory.
While they were mostly the possession of women, there are examples around of those carried by doctors, engineers and draughtmen. I recently saw an engineer's étui or pocket drawing case on eBay complete with brass instruments and boxwood rulers in shagreen from the early 1800s.
Étuis were made of a wide variety of materials including gold, silver, ivory, mother of pearl, agate, tortoise shell, wood, leather and shagreen (shark or ray skin). I've even seen one set inside a large hinged walnut shell. I wish now I had bought it.
Some are decorated with etched designs and some in gorgeous enamels as you can see in these photos.
Even the shapes are delightful. The tapering enamel ones are my favourites and I would love to own one. Others were much more elaborate rococco affairs.
A trawl through eBay came up with a variety including a number from the Palais Royal - these items are always of beautiful quality.
I've seen larger cases from the late 1800s described as étui although they look much more like the modern dressing case or manicure set.
You can still buy little sewing cases which serve a similar purpose today as the étui. I have two tiny modern ones - hinged boxes containing sewing items.