Onyx is a banded form of Silica - in fact very similar to Agate and differing only in the form of bands where Agate has curved bands and Onyx has parallel bands and the  colours of the bands are very varied.

Having a hardness of 7 on the Mohs Scale, it is relatively hard but still able to be used for carving

See Below for a few examples of Onyx in various forms.