Clarity refers to the internal and external birthmarks of the gemstone. Similar to fingerprints or snowflakes, it’s these naturally occurring inclusions that make each gem in the world unique. As indicated in the chart below, the quantity inclusions in a gemstone increase as you move from left to right. The grading is based on 10 power magnification. The usual benchmark in deciding on clarity is whether you can see the inclusion with the naked eye.
Simply stated, an accurately graded I1 will show inclusions fairly easily. An SI2 will show the inclusions with difficulty (remember that the inclusion will be much easier to see in normal wear when it is dirty). Anything above these grades, if properly graded, indicates the diamond has fewer inclusions. Diamonds below these grades will easily show the inclusions.
Be sure to ask your jeweller for their credentials. There is no governance in Canada that states one needs to have any formal training to work in or own a jeweller store, or provide appraisal services. Any jeweller's training is voluntary - ask what kind of formal training your jeweller has.