I have written about this in 1998.
Pietra dura is a mosaic ornamention in which little pieces of
gemstones are put side to side to form a mosaic. These stones
are layed in a flat gem like f.i. onyx. They make holes in the
onyx with drops of accid (or this is the original manner) You
are excpected to see flowers, ruins, castels etc. A typical
Florentine type of ornament.
Mille-fiori is the Roman variety in which little collored
brick-like pieces of glass "tesserae" are cemented side to
side and so creating a mosaic.
Intarsia is sometomes used for Pietra-Dura but is original a
woodcarvers term and is similair but then in wood.
Inlaying is the term used for any metal-in-metal inlay work
(not niello or enamel).
If you have any questions, ask me