Color – Diamond color varies across a spectrum, from black to brown to yellow to white and everything in between. In today’s market, slight differences in color can dramatically affect industry value (white, colorless having the most value). Value is relative, of course, and color is one way to bring uniqueness to a piece. Common color grading notations you’ll see are G, H, I, J and K.
Clarity – Graded under 10x magnification, clarity refers to the absence of internal inclusions and external blemishes. These include white clouds, carbon patches, air bubbles, streaks, feathers, and internal cracks. Common clarity grading notations you’ll see are Very Slightly Included (VS1, VS2) and Slightly Included (SI1, and SI2).
Cut – The diamond is known for its exquisite ability to transmit light, enhanced by advanced cutting techniques combining proportion, symmetry, and polish. These factors contribute to three main elements: brightness (internal and external white light reflections), scintillation (sparkle), and fire (the scattering of white light into the colors of the rainbow).
Carat – Weight is typically referred to in either carats or points. A metric carat is divided up into 100 points. So, a diamond weighing in at 0.5ct can be referred to as a half a carat or a 50 pointer. Large diamonds are naturally more scarce than smaller diamonds. Price increases with weight, and exponentially so. 
For Ceremonial, Commemorative, and Engagement pieces, specific stones are recommended with greater enthusiasm, with longevity, strength and quality in mind. The Mohs Scale of Hardness is the universal grading tool used to measure the resistance of a mineral to surface scratches. Gemstones are rated on a scale of 1 (softest, least resistant) to 10 (hardest, most resistant). Stones with a rating of less than 7 are considered risky business for pieces that must withstand the wear of decades without the need for repolishing. Here’s a selection of popular stones to acquaint you:

Diamond – 10
Sapphire – 9
Ruby – 9
Emerald – 8
Topaz – 8
Morganite – 8
Garnet – 7
Tourmaline – 7
Amethyst – 7
Quartz – 7
Onyx – 7
Turquoise – 6
Opal – 6
Moonstone – 6
Pearl – 3 
Amber – 2