Well there is Raw Sienna and Raw Sienna and this one basically is the finest and rarest, mined from Mount Amiata in Tuscany (pictured left), the mine this was sourced from is now shut and this pigment is from the 1950's.
Why is that a good thing? The seams of this particularly fine Sienna are now exhausted or not economically viable and in my opinion Sienna of this grade is now no longer available, Mount Amiata is also very likely to be the source of the Italian Renaissance Siennas used by Caravaggio, Titian, etc.
It has a chroma (the amount of colour) higher than I've ever seen in a Sienna, in masstone (thick and straight from the tube) it looks like the fairly standard creamy light brown, however when applied thinly luminous earthy yellows appear and for me it is that luminosity which makes it stand out. I mean when using paint like this you realise why the indirect painting of the Renaissance occurred why wouldn't you glaze with colours like these.
Pigment - PBr7 - 1950's production
Opacity - Semi-Opaque
Binder - Linseed Oil