Why could one repose little confidence in a diamond bought on the street

Because of the fake diamonds market

Explanation:

Distinguish between fake and real diamonds is hard work for the inexperienced eye, this is why a lot of the diamonds found on the street are false; fake diamond sellers use the inexperience of the regular buyer to trick them. In addition to this, most of the countries lack strict regulations to avoid this kind of situation, so the best way to purchase a diamond is in a certified store, where you easily report any anomaly, and you can expect a refund if you are not satisfied with your purchase.