Rubies are extremely strong, registering 9 on the Mohs scale of hardness.
Rubies are found in shades of red, from rich darkish red to pigeon blood red and pinkish red. The Burmese coined the term "Pigeon's blood" for rubies that exhibit a rich red with a purple secondary hue.
Rubies are in the same gem family as sapphires. And a ruby that is very pink is actually a pink sapphire. Both rubies and sapphires are in the same crystal family called corundum.
So what's the difference between rubies and pink sapphires?
The line between pink sapphires and rubies can be fine. Scientifically, rubies and sapphires are simply varying amounts of corundum with trace elements like iron, titanium, and chromium. These elements are what create the colors found in corundum crystals: grays, browns, yellows, greens, blues, purples, reds, and pinks.
But the defining difference between pink sapphires and rubies is the presence of chromium which gives rubies that rich red hue. The value of the color of a ruby can be quantified by the hue, lightness, and saturation. If a stone is a rich red but is too dark, that can be undesirable. The whole science is actually largely subjective. Before the 20th century pink was considered "light red" and some rubies described as "pink rubies" would now be considered pink sapphires.
Originally, the finest rubies were mined in Myanmar and it is from there that the term Burmese ruby was born. Almost all rubies have flaws. Rubies without imperfections are rare and command prices even higher than diamonds. Fine-quality rubies over one carat are very rare and the price goes up significantly as size increases.
Rubies are often cut into oval, cushion, and cabochon shapes.