Gem dealers, diamond manufacturers and retailers are looking into on of the last frontiers in diamonds - the "fourth C": cut, which affects brilliance the most.

The roles that color, clarity and carat weight play in diamond value are generally understood. The same cannot be said for diamond cut. The American Gem society Laboratory took the lead in including a cut grade on its diamond reports when it opened in 1995. Now the Gemological Institute of America has undertaken a major study of diamond cut. Some other labs around the world also have turned their full attention to the importance of cut.

What's all the fuss about cut? A diamond's cut - or "make", as some in the trade call it - transcends the other "Cs" because of the way it directs light through the diamond. Even a big diamond or one with good color can look lifeless if light enters and then leaks out because of poor cut. Customers can notice the difference, though they may not know what the difference is.

Here are some terms you will hear in the months ahead as attention to cut intensifies and attempts to "quantify" cut characteristics are developed.

The amount of light returned to the eye, or brilliance, depends on how well the diamond in question reflects and refracts light. This includes dispersed wavelengths, which are reflected from the internal surfaces of a diamond and returned to the eye.

Dispersion is the occasional flash of color, or fire, particularly when viewing a diamond in strong, direct light. Geologically, dispersion is the separation of white light into spectral colors, each of which vibrates at a different frequency. Dispersion flashes may be seen as any color.

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The return of light when it strikes a polished surface is called "reflection". About 17% of the light striking the external surface of a polished diamond verticaly is reflected back into the air; the greater part enters the stone.

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This refers to the flashes of light, or the sparkle, you see when a diamond is moved in the light. Scintillation is best when the clarity characteristics of the diamond are purest. Proper cutting angles and polish of the diamond also contribute to the diamond sparkle.

Luster is the perceived reflection and surface appearance of a polished diamond in reflected light. Generally speaking, the higher the refractive index of the gem, the higher its luster. For example, pearls and amber have lower luster and refractive indexes than diamonds and rubies.

Luster is described as metallic (the highest luster), adamantine (used specifically for diamonds), subadamantine (which refers to such gems as rubies), vitreous, subvitreous, resinous (which refer to such gems as amber), waxy,dull, silky or pearly.