Celestite (strontium sulphate) is an attractive mineral, forming well-formed crystals with a distinctive soft blue hue, but can also be colorless, yellow and tints of red, green and brown.
It has a vitreous lustre and a hardness of 3 - 3.5 on Mohs scale.
Celestite presentation is mostly in pinacoid crystals with small prism faces and two large pinacoid faces at bottom and top. Rhombic crystals can result from diminished or absent pinacoid faces. The habit is quite common in Madagascar specimens. They may also be nodular, fibrous, or granular.
Some crystals may have a uniform color, but many have lighter and darker hues of blue. Pure Celestite is colorless, but its impurities give this mineral a range of colors, including the celestial blue color.
Madagascar is famous for its abundance of shimmering, deep sky-blue Celestite crystal geodes.