Several parts combine into one character.

In oracle bone script, 質 shows two axes (斤 斤) above a cowrie shell (貝, ancient currency). The picture is starkly economic — pledging a shell as collateral, with two axes standing as witnesses to the binding contract. The original sense was "to pledge / give as security." From this root, the meaning fanned out across centuries into a remarkable spread: "essence / substance" (the underlying real value), "quality / nature" (intrinsic character), "to question / interrogate" (demand verification of substance), and "hostage" (a human pledge). One character, four meanings, all rooted in the same idea of "what is fundamentally there." Mainland simplified to 质.

Korean reading "jil." Spans every register: 品質 (pumjil, quality of goods), 物質 (muljil, matter / substance), 性質 (seongjil, nature / property), 體質 (chejil, physical constitution), 質問 (jilmun, question), 人質 (injil, hostage). The same character does industrial QA, philosophy, biology, and crime news.

Mandarin zhì, 4th tone (simplified 质). 质量 (zhìliàng, quality), 性质 (xìngzhì, nature), 物质 (wùzhì, matter / material). The "question" sense survives in 质问 (zhìwèn, to interrogate sharply) and 质疑 (zhìyí, to challenge / cast doubt).

Japanese has two on-readings: シツ (shitsu) is the workhorse — 質問 (shitsumon, question), 性質 (seishitsu, nature), 品質 (hinshitsu, quality), 質量 (shitsuryō, mass in physics). チ (chi) is rarer, surviving in 言質 (genchi, verbal pledge). 人質 (hitojichi, hostage) blends on + kun in a way unique to Japanese — 人 (hito) + 質 (jichi).

Memory aid: two axes guarding a shell — what was pledged is the essence, the substance, the quality.