Gypsum

Gypsum has a very low thermal conductivity (hence its use as an insulating filler). A crystal of gypsum will feel noticeably warmer than, for instance, a crystal of quartz.

gypsum (rosette).

Gypsum is one of the more common minerals in sedimentary environments. It is a major rock forming mineral that produces massive beds, usually from precipitation out of highly saline waters.

Chemical composition - CaSO ₄-2(H ₂O)
Hardness - 2
Specific gravity - 2.3
Transparency - Transparent to translucent
Colour - Usually white, colourless or grey, also shades of red, brown and yellow
Streak - White
Lustre - Vitreous to pearly (especially on cleavage surfaces)
Cleavage/fracture - Perfect in one direction, imperfect in two others / uneven (rarely seen)
Crystal habit/mode of occurrence - Tabular, prismatic (bladed or blocky with a slanted parallelogram outline), fibrous / massive, granular