Labradorite

The labradorite cabochons at the jeweller's looked unremarkable at first sight, like scattered grey pebbles in a tray. Yet they shone with all the colours of the rainbows when the light hit them the right way.

Demosntration of adularescence in labradorite (source)

Labradorite is (Ca,Na)(Al,Si)₄O₈
Starting price by the jeweller called Christopher: Rs 500 per carat. Smallest stone in tray was 15 carats, largest maybe 60 carats or something like that.

Topazes also present, really big ones that grow larger than someone's fist, but their refractive index is barely past the level of silicate glass i.e. the back facets are transparent.