The Collaborative International Dictionary of English v.0.48:

Semiprecious \Sem`i*pre"cious\ (s[e^]m`[i^]*pr[e^]sh"[u^]s), a.
   Somewhat precious; as, semiprecious stones or metals. Used
   mostly of gemstones used in jewelry, such as amethyst,
   garnet, or iolite, which are sufficiently rare to have
   commercial value, but are not considered as precious, as
   are the diamond, emerald, and ruby.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

semiprecious
    adj 1: used of gemstones having less commercial value than
           precious stones; "such semiprecious stones as amethyst,
           garnet, jade, and tourmaline"