How many times have you asked yourself, what difference is there between a diamond and a brilliant? This small doubt is due to the fact that, erroneously, we use the word brilliant to refer to all diamonds.
A diamond is a mineral in the rough (as it is extracted from nature) formed by crystallized carbon. To shape them, a type of carving is applied to them and it is at this moment that they are given a name. The most common and appreciated size is round, called brilliant cut.
The brilliant cut gives the best light refraction angles. Thus, cut diamonds sparkle at their best.
In summary, it would be correct to say that they are brilliant cut diamonds if they are round in shape. Diamond is the material and its size is brilliant.