One stone that is highly underrated is the garnet. Usually, garnets are used as cheap substitutes for rubies, and are very dark. Sometimes they have a deep glow in them, but mostly they are just dark.
Spessartine (Named after Spessart in Bavaria, Germany) garnets have a glorious flamey sparkle that has to be seen to be believed. They vary quite a bit in colour. If I remember correctly the orange ones (mandarin garnets) are most in demand, but my favourite is the neon red. The sparkle is fantastic.
These garnets used to be very rare, but Spessartine garnets are now found in many countries – for example China, Madagascar, Brazil, Nigeria, Tanzania, the USA, Mozambique, and Namibia, Interestingly, I was always told that Spessartine is the correct name, but you find more if you Google Spessartite.
They are a bit hit and miss if you buy online, because they vary so much in terms of quality, colour and sparkle. Having said that, I really lusted after this one on Etsy a while back. It is probably one of the most beautiful I have ever seen. Sadly for me (but happily for my wallet) it was sold before I could pounce. You can really see the sparkle in this one, even in a photograph. Quite different from the very dark garnets you usually see for sale.