The florescence can be yellow or blue or make a stone appear cloudy in sunlight. The easiest aspect of a diamond to understand is its carat weight. However, people commonly associate carat weight with a certain size of diamond. To some extent this is true, but there are other factors that affect how big a diamond looks, notably cut. That is, diamonds twice as big will cost much more than twice as much.
Larger diamonds are rarer and therefore command a higher price.
One carat is equal to 0.20 grams. The Metric Carat was established in the early 1900's and is still used today. The size or carat weight of a diamond also affects the value of a diamond. The larger a diamond, the more valuable it is if all other factors are equal. The larger the diamond, the rarer it is and the higher the value.