Shape describes the external form and face-up outline of the diamond, as well as the cutting style used for the facet arrangement.
The unit of measuring the weight of diamonds. One carat is equivalent to 200 milligrams or 1/5th of a gram. One carat can also be divided into 100 'points'. A 0.75carat stone is the same as 75 points or a ¾ carat stone.
In most diamonds, the colour grade actually refers to the absence of colour. The less colour in the stone, the more desirable and valuable it is.
Clarity is the relative absence of inclusions and blemishes. Clarity grades are based on the number, size, relief, and positions of inclusions that can be seen under 10x magnification.
There is not one single set of proportions that will ultimately define a well cut diamond; it requires measuring a wide range of varying parameters and factors in a number of different combinations to determine a diamonds cut grade.
Note - An internationally accepted system for evaluating the Cut Grade of fancy-cut diamonds (any shape other than Round) does not exist at this time.
Your decision regarding how much to spend on either a diamond or diamond set jewellery should reflect the importance of the occasion, your financial standing and your price limitations.