Goldstone's sparkle can have 2 sources:
- "Real" goldstone is supersaturated with copper (gold) or chrome (green). The sparkles are tiny flakes of metal that precipitated out of the glass (like sugar coming out of hot tea as it cools).
- Other colors of goldstone and "shampoo glass" are made by adding mica powder to the glass.
Encasing should not affect mica-based glass - as long as you don't heat it so high the mica burns up.
But when you encase and pull a thin stringer of "real" goldstone - you may be diluting the copper so that the sparkles disappear. The excess copper is just absorbed in the clear glass.
Can you pull pure goldstone stringer and compare its sparkle to the encased?