When we talk about gemstone identity, there is a difference in terminology definition if using the words "species" or "variety". A gemstone species has a specific chemical composition and crystal structure that the members of that species would all have. In contrast, some species have members that differ from one another in certain ways. A variety is defined by the visual color or optical phenomena of a member within a gem species.
What some people don't know, is that rubies and sapphires actually belong to the same gem species called corundum. Corundum is basically aluminium oxide that has crystallised in the trigonal crystal lattice system. It's pretty hard at a 9 on the Moh's scale, and at its most basic composition- is actually colorless. What makes rubies red and sapphires blue though?
Rubies get their red color from the presence of a trace element called Chromium. The percentage of chromium is very small, just a few parts per million actually, but this interacts with light and the surrounding atoms to produce various degrees of red and pink in corundum. Pink sapphire is also often primarily colored by chromium.
Sapphire on the other hand, usually refers to blue sapphire when used as a standalone term. Blue sapphire mostly gets its color from the presence of iron and titanium. Despite this, some people don't know that there are actually many different colors of sapphire- such as green, pink, purple, and orange. These are often called 'fancy sapphires' or are preceded by their respective color hues.
Corundum represents one of the most commonly used colored gemstones in the jewelry trade today. Rubies are traditionally the most expensive colored gemstone actually. Sometimes though there is debate over where the line holds clear in the definition between rubies and pink sapphires, as the only difference is in the strength and presence of the red color component. If a stone's color is too light, traders usually term it as pink sapphire instead of ruby.
