How to identify rubies

Ruby has been regarded as a treasure since ancient times. In European mythology, the birthstone in July is a ruby, which is a symbol of love. Ruby is often regarded as a witness of marriage at the wedding of European royal families. There are countless romantic legends related to rubies, and the Bible also believes that rubies and wisdom are related to beauty. So why does ruby become the pursuit and yearning of so many people? Today, let's unveil the mystery of ruby ~

Figure 1- Red gem ring (Source: Baidu Picture)

Ruby refers to red corundum, and the best ruby color is "pigeon blood red". The molecular formula of corundum is Al2O3, and pure corundum is colorless. In natural corundum crystals, a few elements such as chromium, iron, manganese, titanium, nickel, cobalt and vanadium often replace aluminum in the form of isomorphism, which makes the crystals appear in various colors. Corundum containing Cr is red, namely ruby; Corundum containing Fe and Ti will be blue, Ni will be yellow and Co and V will be green. All colors of corundum except red corundum become sapphires, and the front can be decorated with colors, such as pink sapphires, yellow sapphires and green sapphires. Some jewelers will classify pink corundum as ruby sequence, which is not strict in mineralogical classification.

Figure 2-Natural sapphire single crystal, semi-automorphism (source: Baidu picture)

Figure 3- Natural green sapphire single crystal, which is automorphic (image source: Baidu image)

Figure 4-Natural pink sapphire single crystal, special-shaped crystal (source: Baidu picture)

Figure 5-Natural yellow sapphire single crystal, special-shaped crystal (source: Baidu picture)

Corundum is the second hardest mineral in nature, and its Mohs hardness is 9, second only to diamond (Mohs hardness is 10), which is also commonly known as diamond. Because corundum has higher hardness and lower price than diamond, it is often used as a substitute for diamond, as sandpaper or grinding material. Corundum is glass luster to sub-diamond luster with high reflectivity. Corundum minerals belong to the cubic crystal system, and most of the crystals are barrel-shaped and columnar, and a few are plate-shaped parallel to the (00 1) plane (Figure 6). Corundum crystals have no joints (Figures 2, 3 and 4), but cleavage develops (Figures 2 and 5). Ruby, as a kind of corundum, is hard to escape, so it is often called "Ten Hong Jiu Cracks". When there are fine inclusions in mineral crystals (see below), or when external force is applied, cracks appear along the weak plane of crystal mechanics, that is, cleavage. Cleavage is not an inherent property of minerals, nor does it strictly follow the lattice structure.

Figure 6- Optical orientation of corundum crystals under ideal conditions (source: crystallography and mineralogy)

Natural rubies in nature often contain various inclusions, which affects the value of rubies as gems to some extent. Natural rubies with little cleavage and transparency are extremely rare. When ruby contains a large number of fine needle-like rutile inclusions, and rutile is distributed on 000 1 plane just along the intersection of crystal planes, and crosses each other at 120, the ruby is machined into a cambered surface along the crystal c axis (Figure 6), and the starlight effect can appear.