This is a natural dark blue Sapphire crystal.
Sapphire derived from Sanskrit "ShaniPriya" is a typically blue gemstone variety of the mineral corundum, an aluminium oxide. Trace amounts of elements such as iron, titanium, chromium, copper, or magnesium can give corundum respectively blue, yellow, purple, orange, or green color. Chromium impurities in corundum yield pink or red tint, the latter being called ruby.
Commonly, sapphires are worn in jewelry. Sapphires may be found naturally, by searching through certain sediments (due to their resistance to being eroded compared to softer stones) or rock formations.