In what type of geological formation is the majority of gem-quality topaz found?

Gem-quality topaz is primarily found in pegmatitic veins. Pegmatites are intrusive igneous rocks that typically form in the final stages of magma crystallization. These formations are characterized by their large and well-formed crystals, which occur because they have had more time to grow than crystals in regular igneous rocks.

In the context of topaz, pegmatitic environments are rich in the necessary elements, particularly aluminum and fluorine, which are essential for the formation of topaz crystals. The slow cooling of the magma allows for the diffusion of these elements, contributing to the creation of desirable gemstones with greater clarity and aesthetic appeal. Such large crystals often contain fewer inclusions and can be quite vibrant in color, which makes them highly sought after in the gem market.

In contrast, alluvial deposits typically consist of materials that have been eroded and transported by water, often resulting in smaller and less pristine gemstones. Drafts and basalt flows are not conducive to the formation of topaz, as they lack the specific conditions that lead to the formation of high-quality gems.