Verneuil (Flame-Fusion) Process

 

Melt-Growth Processes

Verneuil (Flame-Fusion) Process

Various manufacturers (all colours, including six-rayed stars).

Identification Characteristics:

1. Curved growth lines (thin striae or bands), seen best at roughly 90° to the boule’s length. These curved lines are not concentric.
2. Gas bubbles, round or elongated at 90° to direction of growth lines, from pinpoints to large distorted doughnut-shaped spheres or highly irregular worm-like distortions. They are usually distributed in clouds which follow the curved growth structure of the boule. In blue stone, the gas bubbles may show concentrations of blue colour around them.
3. 1. Polysynthetic twinning along the c-axis (‘Plato lines’), seen between crossed polars, most effectively when the specimen is immersed, in sets of one, two or three directions.
   2. Polysynthetic twinning along the rhombohedron, sometimes with accompanying boehmite needles identical to natural corundum.
4. Induced fingerprints and feathers entirely similar to natural corundum.
5. Traces of the seed rod or seed crystal. Found at the base of the boule and featuring frosted surfaces at the seed junction.
6. Some varieties may show useful UV fluorescence. The V-doped colour change type shows a diagnostic line at 475 nm. The full Fe spectrum (451, 460, 470 nm) is not seen.
7. Irregular colour distribution and rounded facet junctions of blue varieties, in particular, may cause confusion with surface diffusion-treated corundums.
8. Dense clouds of extremely fine, exsolved rutile silk in star material. The clouds do not show the angular zoning patterns that are common to natural stones. Instead, they may display curving bands (which are sometimes concentric).

Related articles

• Czochralski (Pulling) Process
• Floating-Zone Process
• Simulants and Imitations of corundum

Further reading

Synthesis and Simulation in corundum