Amora Gem and Diamond are related crystals.
Since most people are familiar with diamond, we can explain Amora Gem by starting with a pure diamond. By taking a pure diamond, which is 100% pure carbon, and then removing every other carbon atom and putting a Silicon atom in instead, we end up with an Amora Gem's atomic composition (50% Carbon, 50% Silicon).
Because Silicon is a larger atom, the 50% carbon atoms have to shift to make room, but after the carbon atoms make room, you now have an Amora Gem.
(Note that to do this and grow an Amora Gem requires very extreme heat. These temperatures are found only around large stars or in extreme growth chambers that create the star-like environment. Diamond itself would vaporize in these star-like conditions which is why Amora Gem was only made available on Earth recently).
The 50% Silicon atom composition is what gives Amora Gem superior fire, brilliance and lustre versus diamond. Silicon is larger than carbon, so it is better able to bend light, and light bending and dispersion (measured as refractive index and dispersion index) is what provides a gem's brilliance and fire.
This is similar to why lead crystal is more sparkly than regular crystal - the addition of the larger lead atoms provide more light bending versus pure crystal, and thus greater beauty.
Similarly, the shifting of the carbon atoms eliminates the 'seams' that are inherent in diamond. These seams, known as 'cleavage planes' are effectively atomic zippers - hit there and your resistance is only one carbon atom bond all the way down. This is why diamond can be tough in most directions, but extremely weak at certain points.
By contrast, the alternating sizes of the carbon-Silicon-carbon structure in Amora Gem ensures that there are no zippers in the crystal, and so it is tougher than diamond. Toughness is resistance to chipping or fracture.
Thus, by mentally building an Amora Gem starting from diamond, you can now see both what an Amora Gem is (50% carbon, 50% Silicon crystal), how it is a star-borne cousin to diamond, and why Amora Gem has superior gem beauty and toughness.