Many of the fossilised dinosaur eggs which have been found have significant surface detail - sometimes called 'decoration'.
The surface texture has been classified into six different types (see Wikipedia), which can in some cases be used to distinguish the species.
The purpose of the 'decoration' isn't known - and coming to a conclusion is made more difficult by the fact that nearly all eggs seen today (birds, reptiles etc) are smooth-shelled.
One proposal is that the ridges etc. helped to regulate gas exchange (especially CO2) with the atmosphere, but this has not yet been proven.
Further detail (with photos) Berkeley University
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