Sodalite is a dark blue, semi-transparent gem that is often substituted for lapis lazuli. It doesn't have the gold flecks that are specifically distinctive of real lapis, though the calcium veining is found in the stone. Unlike real lapis, which is completely opaque, sodalite will let some light through it. The lack of gold flecks and the semi-transparency should allow you to differentiate between real lapis and this common substitute.