Sapphire Quality Factors

Several types of inclusions are found in sapphires. Among these are long thin mineral inclusions called needles. Fine needles are called silk when they occur as the mineral rutile in intersecting groups. Other clarity characteristics in sapphire are included mineral crystals, partially healed breaks that look like fingerprints, colour zoning, and colour banding.

Rutile needles clearly seen in this Sri Lankan Sapphire.  They do not detract from it's beauty

Generally, inclusions make a stone less valuable. Price can drop substantially if the inclusions threaten the stone’s durability. Even so, inclusions can actually increase the value of some sapphires. Many of the most valuable Kashmir sapphires contain tiny inclusions that give them a velvety appearance. They scatter light, causing the coveted visual effect without negatively affecting the gem’s transparency.

Star rubies and star sapphires belong to the phenomenal corundum category. The star effect is called asterism. It’s caused by reflections from tiny, needle-like inclusions that are oriented in several specific directions. Stars are usually made up of 2, 3, or 6 intersecting bands, resulting in 4, 6, or 12 rays.

Star Ruby with 6 clear rays

The shape of a rough sapphire crystal influences the finished stone’s shape and size. Rough sapphire’s most common crystal form is a barrel- or spindle-shaped hexagonal pyramid. For this reason, finished sapphires are often deep.

To achieve the best overall colour, maintain the best proportions, and retain the most weight possible, cutters focus on factors like colour zoning, pleochroism, and the lightness or darkness of a stone.

Color zoning—areas of different colours in a stone—is a common sapphire characteristic. Blue sapphire often has angular zones of blue and lighter blue. To accommodate colour zoning in some sapphires, cutters orient the concentrated colour in a location that offers the best visible colour in the cut stone.

Colour zoning shown clearly in this Australian Sapphire

In Sri Lankan sapphires, the colour is often concentrated close to the surface of the crystal. If a cutter can orient the culet within the concentrated area of colour, the stone will appear entirely blue in the face-up position.

Pleochroism is different colours in different crystal directions. Blue sapphires often have greenish blue and violetish blue pleochroism. It’s most desirable to orient the cut so the stone shows the violetish blue colour when it is set in jewellery.

Article from:  GIA Education