Lapis is not a mineral but a rock: a mixture of the blue mineral lazurite with white calcite and metallic pyrite. The classic material has a deep, slightly violet blue with a light scatter of gold — the pyrite. Historically it was ground for pigment, which is where the word ultramarine comes from.
The white calcite flecks are the feature to look for in judging natural material: high-grade lapis has very little, and a strand sold at a low price as "top grade" with no white at all is usually dyed. This is the standard commercial grade, with both calcite and pyrite visible.