Diamond is not only harder than any other natural gem, it’s also a lot harder than any natural or manufactural simulant. Diamond’s exceptional hardness makes it possible to achieve very sharp facet junctions and superior polish.
A simulant’s comparative softness means that its facet junctions always appear more rounded than a diamond’s. And most simulants quickly show signs of wear, with scratches and visible abrasions.
Diamond’s capacity for a superior polish gives it much higher luster than a simulant. Even a new, well-polished simulant can’t equal the luster of a clean diamond in good condition.