Cleavages that are small and internal are not as serious as those that break the surface which could cause damage to the diamond should it be struck there. From Flawless to VS1 it is very difficult if not impossible for the average person to see the inclusions even with the aid of a 10x loupe. SI1 is a very good clarity that will almost always require a loupe. Some diamonds with a clarity rating of I1 may even be eye clean and acceptable for an engagement ring. The clarity scale goes to I3 where at the bottom of that grade a diamond could be virtually opaque, perhaps better suited for industrial use.
Most people buy a diamond for an engagement ring that is eye clean and good color.
You really don't want a diamond that is dingy in color or one with a large black spot or crack in it. J. The clarities that we sell the most of are SI with some in the VS range and some in the I1 range. You may want to keep shopping if the salesperson cannot give you an accurate color, clarity or weight of the diamond you are buying. Try to find a store that will sit down with you and explain what is going on with diamonds. Be sure to compare colors and look under magnification so that you can understand clarity.