A quantum dot includes: a core including at least one first positive ion precursor and at least one negative ion precursor; a shell including at least one second positive ion precursor and at least one negative ion precursor and wrapping the core; and a ligand formed on a surface of the shell, wherein the first positive ion precursor is an n-period element and the second positive ion precursor is an (n-1)-period element, where n is an integer of 3 to 6.

A quantum dot includes: a core including at least one first positive ion precursor and at least one negative ion precursor; a shell including at least one second positive ion precursor and at least one negative ion precursor and wrapping the core; and a ligand formed on a surface of the shell, wherein the first positive ion precursor is an n-period element and the second positive ion precursor is an (n-1)-period element, where n is an integer of 3 to 6.

Provided is a photochromic substance that has lower toxicity, exhibits good sensitivity in a visible light region, changes color deeply, has slow speed of color fading, has chemical and thermal stability, and has good durability. The photochromic substance has a composition represented by the formula:
Ba.sub.(a-b)Ca.sub.bMg.sub.cSi.sub.dO.sub.e:Fe.sub.fM.sub.gM.sub.h where 1.8a2.2, 0b0.1, 1.4c3.5, 1.8d2.2, e=(a+c+2d), 0.0001f, 0.0001g, 0h, M is at least one of Al and Eu, and M is at least one element selected from the group consisting of Na, K, Nd, Li, S, C, Ti, V, Mn, Cr, Cu, Ni, Co, Ge, Zn, Ga, Zr, Y, Nb, In, Ag, Mo, Sn, Sb, Bi, Ta, W, La, Ce, Pr, Nd, Sm, Gd, Er, Ho, Tb, Tm, Yb, Lu, P, Cd, and Pb.