Which of the following gems is a cryptocrystalline aggregate?

Chalcedony is classified as a cryptocrystalline aggregate because it is composed of very small crystals that are not visible to the naked eye, giving it a smooth and waxy appearance. This characteristically fine-grained structure is created through the accumulation of silicon dioxide (SiO₂) and results in a homogeneous, translucent stone that may exhibit banding or other patterns depending on its formation process. The term "cryptocrystalline" indicates that the crystals are so small that they can only be identified through advanced microscopy techniques.

In contrast, the other gems mentioned in the options exhibit different structural characteristics. Amethyst, while a variety of quartz, is typically macrocrystalline and features visible crystals, commonly found in a larger, well-formed structure. Topaz and sapphire also have a crystalline structure where individual crystals can be identified, placing them outside the category of cryptocrystalline aggregates. Therefore, chalcedony stands out as the correct answer due to its defining properties of being a cryptocrystalline aggregate.