How To Identify Synthetic Corundum Produced By The Verneuil Process (Flame Fusion Method)

Synthetic and natural corundum will have the same physical, chemical and optical properties, but with synthetic flame fusion corundum there may be a slight overlap. The identification is based on the detection of inclusions with loupe or microscope and good lighting; fluorescence may be a complimentary test.

First look for:

Color

This is an indication only.

- The coloring elements in synthetic corundum may not be the same as in natural stones.

- Natural corundum is usually cut with the table facet at right angles to the optic axis where the best color is, but in synthetic corundum the stones may be cut in different orientation because of the size and shape of the rough gem material.

Clarity

- Usually synthetic corundum is produced to cut eye / loupe clean stones, but the presence of spherical gas bubbles is immediate proof of synthetic origin. Depending on lighting conditions, small bubbles tend to appear black, while larger ones have a dark border and a light center. The stones may also have unmelted alumina particles that look like tiny crystals.

- The presence of curved color banding and curved growth lines is adequate to confirm the stones as synthetic, because in natural corundum growth bands and lines is either straight or angular in shape following the outlines of the crystal faces. Curved striations are not the result of uneven color distribution, but due to the result of rotating boule (the rough gem material).

Note: Heat treatment at high temperatures has been carried out on Verneuil (flame fusion) synthetics to enhance color and appearance, in which case curved growth lines are difficult to see or may be nearly absent. Natural looking fingerprints can be produced by quenching to produce near-surface cracks; then by subsequent heating ('healing') in a flux.

Cut

Natural corundum is usually cut with weight retention in mind because of the high value of rough gem material. Synthetic corundum may be well cut; also look for inexpensive cutting styles like scissors or cross cut, and fire marks due to rapid polishing. Occasionally synthetic corundum may be cut asymmetrical to duplicate native cutting—Translation: cut like a natural corundum. The stones may also show packetworn appearance or abraded facets due to a large number of stones rubbing together.

Size

How often do you see 5 carat, 10 carat, 15 carat, 20 carat, 40 carat, or 50 carat flawless rubies or sapphires?