Ultramarine Blue was historically made from Lapis Lazuli that came from Afghanistan. The raw mineral was ground and washed and separated into various grades until a pure blue pigment was achieved. This pure blue was so expensive that only rich patrons or the church ever requested its use. Today true Ultramarine is still available but it is still very expensive-- as the saying goes "the more things change... the more they stay the same".
That's why today we have a synthetic equivalent. Whereas in the past blue was hard to come by today it is easily accessible to artist and designers alike. Ultramarine comes in a variety of shades from deep dark blue to violet to pale pink. They are all completely lightfast, compatible with each other and other pigments. They work in all media except cement or lime plaster.
Currently, there is only one ultramarine blue which works in plaster-- Ultramarine Blue Dark ( DCCR1). It is both acid proof and alkali safe.