The ideal cut is one that reflects light in parallel rays giving that beautiful brilliance edge to edge we all love. Every gem type (more technically every refractive index) has what is called a "critical angle" which is the ideal angle at which light begins to reflect as opposed to refract (pass through). A stone cut with angles less than the critical angles results in a “window” effect, since light escapes out the bottom and sides of the gem, not up through the table. When a stone is cut with angles more than the critical angle, you’ll see “invisible edges” because the reflected light up through the table is focused (like a light beam) towards the center of the table. 

Gems with higher refractive indices are generally easier to facet with good brilliance because these gems have a wider range of angles that will still bounce the light around and ultimately up through the table. Different methods of cutting have varying effects on the brilliance of the end product.

Commercial cutters that use machines to cut have problems with low refractive index gems because they do not adjust the cutting angles. The cutting angles for CZ are often used, and these angles produce an ugly Aquamarine. Machines also have a difficult time producing a consistent polish. The machines can't think about (or see) the gem finish and apply a longer polishing time as the polishing medium deteriorates.

Commercial cutters that utilize unskilled manual labor to produce the cheapest product do not consider angles at all.  By rushing the product through the process they lose the ability to control angles and polish.  Commercial cutters that utilize skilled and experienced labor produce the best quality you can get at a volume price. These cutters consider critical angles, symmetry and polish throughout the process.  If you add good quality control, you end up with very consistent quality gems.

Custom Cutters produce the finest quality gems. They calculate cutting angles exactly and use high definition measuring techniques to maintain superb symmetry.  Gems are cut individually and usually by request. The price is usually very expensive since you are paying an expert for hours (sometimes days) worth of work.