Spinel, the Gem Traders gemstone. Tough, durable, colourful, beautiful. Under-valued by most.
It forms under the same geological conditions as Ruby and the two stones are often found together.. infact the two have often been confused (eg the Black Princes Rudie in the English Crown is actually a Spinel, as is the 'Timur Ruby' in the English crown jewels).
It is an 8 on the Moh's scale of hardness.. so only marginally softer than rubies or sapphires.
Spinel comes in a wide spectrum of colours but it is the redish and pink tones for which the gem is best known..
images 1 & 4 from Secrets of the Gem Trade by Richard Wise, all other images from Wildfish Gems (thanks Audra), where you can purchase many of these stones are more, with the guarantee of all being untreated, non-enhanced natural gemstones.