Lapis lazuli is a semi-transparent to opaque jewel. Its luster, or surface gleam, varies from waxy to vitreous, or glasslike. The gem’s refractive index, or ability to bend light, is rated at 1.5. Diamond by comparison is 2.417. Other optical properties like dispersion and pleochroism are absent.
When compared to an equal volume of water, lapis lazuli’s specific gravity is 2.5 to 3 times heavier. If fractured, the rock produces a conchoidal, or curved step-like pattern. When dragged across a rough surface, lapis lazuli produces a blue streak. These traits help distinguish the gem from other materials.
One of the unique traits of lapis lazuli is its contribution to painting. Ultramarine is a pigment whose name means “beyond the sea,” describing its intense blues. For centuries, its coloring came from ground lapis lazuli.
The first known use of ultramarine was evident in cave paintings in Afghanistan from the 6th and 7th centuries. Technological improvements during the early Middle Ages helped extract more color from lapis lazuli. This involved grinding the gem into a fine powder, mixing it with other ingredients and kneading the combination in lye three times or more. Ultramarine remained so precious it was reserved for choice subjects.
Discoveries in the early nineteenth century lead to the creation of synthetic ultramarine. It required no lapis lazuli and produced color more intense than the original pigment. Synthetic ultramarine was also much cheaper to create, leaving more lapis lazuli for carvings and jewelry.
Ideal Lapis Lazuli
Lapis lazuli comes in a range of blues, including indigo, royal, turquoise and sky. Green also exists. The most prized colors are medium to dark blue in tone, intense saturation and slightly greenish blue to violetish blue in hue. Uniform coloration is sought after and the most precious.
Blue is the preferred shade of lapis lazuli. Calcite and pyrite are common elements of lapis lazuli, but their overabundance means less of the desired hue. While some flecks of pyrite is admired, excess amounts can turn the gem green.
The jewel’s opacity makes it unsuited for faceting. For jewelry, it’s often made into beads, cabochons or inlays. Carvings are another popular way to shape the gem, whether as a wearable piece or an object d’art. Popular subjects include animals and freeform pieces.