Celestite [also called Celestine] is a clear to blueish gray mineral, with crystals which often resemble those of Barite [though Celestite is much less common]. The name comes from the Latin 'caelestis', meaning 'heaven or 'sky' - a reference to the beautiful blue color.
Some of the most impressively blue specimens come from Madagascar, such as the example above.
Celestite is a popular mineral for collectors, for obvious reasons. It is also the only commercially mined source of strontium carbonate and other strontium compounds, which are used in pyrotechnics, television tubes, and even cancer treatments.