This sounds like a trivial question and it is the reason we quote thickness on every product.

What a thick bookmark does

Left in a closed book, it holds two pages apart at one point. The block of pages is no longer flat, the spine is forced to curve around the extra thickness, and over months the binding takes a permanent set.

On a cheaply bound paperback it can crack the glue outright. On a sewn hardback it is slower and still visible.

Which products are worst

Heavy leather straps and thick etched metal, both of which are bought precisely because they feel substantial in the hand.

That is why our leather is a thinner temper with skived edges and our metal is a gauge chosen to flex and return rather than to feel weighty.

The other kind of damage

An etched metal edge left as cut will score the page it sits against, and occasionally a finger. Every edge should be rounded and deburred.

And a magnet strong enough to feel reassuring will emboss a thin page over months, which is why grip is tuned rather than maximised.

What to specify

A caliper or gauge, not an adjective. Rounded corners and deburred edges. A magnet grip that is described rather than boasted about.

If a supplier cannot give you a number for the thickness, they have not thought about the book.