The key distinction is physical form: heading 2505 covers naturally occurring sand-sized particles, while heading 2506 covers quartz in lump, block, or slab form — material that has not been naturally disaggregated to sand size. Crushed or milled quartz that has been artificially reduced to a fine particle size may fall outside both headings depending on the degree of processing.

While purity grade does not change the HTS classification, semiconductor and solar-grade quartz often requires detailed certificates of analysis showing SiO₂ content and trace-element levels. Buyers in those industries typically negotiate specification tolerances in purchase contracts, and customs value declarations should reflect the grade premium. Norway is a notable source of high-purity quartz for these demanding applications.