Understanding Cryptocrystalline Quartz

Cryptocrystalline quartz refers to quartz composed of extremely fine intergrowths of quartz and its polymorph moganite, where individual crystals are too small to be seen with the naked eye. These are often referred to as having a fibrous or granular texture.

Analyzing Quartz Varieties

Let's examine each option in relation to being a cryptocrystalline form of quartz:

• Jasper: A dense, opaque variety of chalcedony (which is cryptocrystalline quartz), often brightly colored. It is a cryptocrystalline form.
• Agate: A banded variety of chalcedony, known for its fine layers. It is a cryptocrystalline form.
• Chalcedony: The general term for cryptocrystalline quartz, characterized by its fibrous or granular texture. It is the definition of cryptocrystalline quartz.
• Amethyst: This is a macrocrystalline variety of quartz, meaning its crystals are large enough to be seen with the naked eye. Its purple color comes from iron impurities. Therefore, it is NOT a cryptocrystalline form.

Conclusion on Quartz Forms

Based on the definitions, Jasper, Agate, and Chalcedony are all considered cryptocrystalline forms of quartz. Amethyst, however, is a macrocrystalline variety.

Thus, the option that is NOT a cryptocrystalline form of quartz is Amethyst.