Lapis Lazuli is a generalised stone name for the easily recognisable blue stone we all know so well. However, it could easily be mistaken for Sodalite or Azurite.
The blue parts of any specimen of Lapis Lazuli is a mineral called Lazurite. Nearly always, Lazurite is accompanied by Calcite and Pyrite. The Calcite gives Lapis Lazuli its white markings, which leaves Pyrite making up those lovely golden specks or streaks.
Lapis Lazuli is graded and priced accordingly and it all depends on how much Calcite and Pyrite is present. Generally speaking, a Lapis specimen is considered more valuable and desirable when little to no Calcite markings are evident. But beware of dyes which can often be used to hide it!