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How did Venetian glassmakers make cristallo glass resemble rock crystal?

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fish of core-made glass — glassware Fish of core-made glass with “combed” decoration, Egyptian, New Kingdom, 18th dynasty (c. 1363–46 bc). In the British Museum. 0.141 m × .069 m.

Venetian glassmakers achieved the appearance of rock crystal in their cristallo glass by focusing on the purification of their raw materials and the chemical neutralization of natural tints.

To create this clear, colorless glass, they utilized high-quality ingredients, often preferring calcined and crushed river pebbles—selected specifically for their whiteness—over common sand. Because primitive glass naturally contained iron impurities that resulted in a smoky or tinted appearance, Venetian artisans learned to eliminate these accidental colorizers by adding pyrolusite, a manganese mineral known as "glassmaker’s soap".1

While the resulting cristallo often retained a subtle grayish or brownish cast, it was considered a significant achievement in transparency for the era. This material was highly valued for its ductility, which allowed glassblowers to create delicate, thin-walled vessels that mimicked the clarity and elegance of natural crystal.23