Imperfection (I1) (I2) (I3)
When we get to the "I" clarity grades, the "I" simply stands for Imperfect. In diamonds that are graded Imperfect, generally you can expect to see it with the human eye without much difficulty (depending on your age and eyesight of course :) I1's contain inclusions that will be visible to the human eye except in circumstances where the inclusions may be many but scattered. Often in I1's, an eye visible feather or crystal inclusion directly in the center under the table facet of the stone is not uncommon. Something that separates I1's from the lower two grades of I2 and I3, is that while the stone will contain an eye visible inclusion or two, the brilliance on the stone overall will not be diminished. The diamond will still be a brilliant stone if the cut grade on it is good. Yes, there are engagement rings that are bought in the world that are Imperfects and there are more stones in this clarity grade on the market than in any other grade. I have had many people purchase "I" grade diamonds who knew exactly what they were getting and are very happy campers. The key is seeing what you're getting. If you get an "I" grade diamond and the major inclusion is off to the side, and most of it can be hidden with a prong or two, and the girl sees the size of the stone she can get for the money, it may be a consideration although I generally will not recommend them.
The I2 and I3 grade diamonds contain inclusions that are not only large and visible to the human eye, but these are clarity grades that have large inclusions that can diminish brilliance and potentially affect the stones durability.
Included category (I) diamonds have obvious inclusions that are clearly visible to a trained grader under 10x magnification. Included diamonds have inclusions that are usually visible without magnification or have inclusions that threaten the durability of the stone. The I category is divided into three grades; I1 denotes a higher clarity grade than I2, which in turn is higher than I3. Inclusions in I1 diamonds often are seen to the unaided eye. I2 inclusions are easily seen, while I3 diamonds have large and extremely easy to see inclusions that typically impact the brilliance of the diamond, as well as having inclusions that are often likely to threaten the structure of the diamond.