University of Minnesota’s Mineral Pages: Gypsum
Gypsum occurs in two main varieties as white earthy masses or as  clear crystals. In crystal form, its softness and single perfect  cleavage with two less well-developed cleavage directions usually serve  to distinguish gypsum from other similar-looking minerals.
Anhydrite:
When gypsum loses its water component, it forms the mineral anhydrite (CaSO4).  When anhydrite absorbs water it becomes gypsum. This ease of exchange  is the basis of gypsum’s use in plaster. Consequently, the two minerals  are very similar to one another and are only easily distinguished by  heating a sample in a closed tube. As it is heated, gypsum samples will  produce fine water droplets, while anhydrite does not. So technically  there is an important difference between the two minerals, but for  non-specialists interested in collecting or appreciating earth  materials, the two can almost be considered as different forms of the  same material.
Halite:
Halite and gypsum both form clear crystals that are very soft and are  easily scratched. Licking a sample will quickly distinguish the two, as  halite has the same composition as table salt and a very distinctive  ‘salty’ taste. For those who dislike the idea of licking mineral  samples, the two minerals can also be distinguished by their cleavage  patterns. Halite has three perfect cleavages that form at right angles  to form cubes. Gypsum crystals only have one perfect cleavage direction.  The other two cleavage directions are not as pronounced, so broken  gypsum crystals tend to form rhomb-shaped fragments, rather than cubes.
Calcite:
Calcite crystals can be distinguished from clear gypsum crystals by  both their cleavage and the ease with which they react with dilute acid  to effervesce or bubble. Calcite has three perfect cleavages that form  rhombic fragments, while gypsum only has one perfect cleavage and two  less distinct cleavage directions so that, when broken, it tends to form  less well-developed rhombs than calcite does.
Dolomite:
Dolomite crystals can be distinguished from clear gypsum crystals by  their cleavage. Dolomite has three perfect cleavages that form  well-developed rhombic fragments when broken. In contrast, gypsum has  only one perfect cleavage direction and two less distinct cleavage  directions. Broken gypsum crystals tend to form less well-developed  rhombs than dolomite does. If part of a sample is crushed into a powder,  the powdered dolomite will also react with acid to produce small  bubbles, while gypsum does not.
Did you know...
From ancient art to walls and modern dentistry, gypsum in the form of  plaster has been widely used throughout human history. Its versatility  is due to the ease with which gypsum can lose or gain water, an ability  tied to its origin. Gypsum, a soft non-metallic mineral, almost  exclusively forms in sedimentary settings, as seawater is evaporated or  as dissolved ions precipitate from groundwater. Its mineral structure  still contains some water though. Heating gypsum will drive off this  remaining water to produce a dry powder that can later be combined with  water to again produce gypsum. Although gypsum can form clear  transparent crystals, it usually occurs as massive white chalky  deposits. It is the most common sulfate mineral and one of the most  widely used non-metallic minerals in the world.
Description and Identifying Characteristics
Gypsum is a very soft mineral that is easily identified by its  hardness, cleavage, and solubility in water. Typically clear to white,  gypsum may be colored reddish to brown or yellow if impurities are  present. Most gypsum occurs in its massive form, as layers of rock that  often intercalate layers of shale, limestone, or dolostone. Alabaster is  a common name used for particularly pure deposits of massive gypsum.  Gypsum also occurs in crystal form, with the two most common varieties  being large clear crystals that are often termed selenite, and fibrous  crystals, known as satin spar. ‘Selenite’ is the Greek word for moon,  and refers to the crystals well-developed pearl-like luster that can  reflect a soft moon-like glow. In arid sandy areas, growing gypsum  crystals may bind sand grains together into complex clusters known as  ‘desert roses’.
 
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In Our Earth: The Geologic Importance of Gypsum
Gypsum almost exclusively occurs in settings where seawater is  evaporating or where groundwater containing dissolved ions from  evaporite rocks reforms these ions as gypsum. As a consequence, gypsum  typically occurs as layers associated with limestone, dolostone, shale,  and rock salt. As seawater evaporates, gypsum is the first ‘salt’ to be  precipitated, followed by anhydrite, halite, and finally sylvite.  Usually found with other evaporite and carbonate minerals, such as  anhydrite, calcite, dolomite, borax, and halite, in altered carbonate  rocks, gypsum may also be found with sulfur and pyrite.
In ground water systems, gypsum can also form at hot springs or as  cave deposits, especially in areas where groundwater has passed through  evaporite sedimentary rocks. To a lesser extent, gypsum may be  precipitated around volcanic vents called ‘fumeroles’, where it is often  associated with sulfur.
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In Our Society: The Economic Importance of Gypsum
Gypsum has been widely used since prehistoric times. The name comes  from  the Greek word ‘Gupsos’, which means ‘to cook’ or ‘burned’, a  reference to how gypsum is commonly prepared for use as plaster.  For  centuries, gypsum has been used for plaster and pigments in both  constructing and decorating buildings. Although plaster may have been  its most important use, Assyrians also sculpted the alabaster variety of  massive gypsum into ornaments and figurines, while Greeks used  transparent selenite gypsum crystals for their temple windows, long  before glass was invented. 
When gypsum is heated, water is driven from its crystal structure to  produce anhydrite. Anhydrite is commonly known as Plaster of Paris,  because thick deposits of gypsum occur in the Paris Basin. When Plaster  of Paris is mixed with water, it forms a paste that can be easily shaped  and that hardens into solid gypsum. This is the source of our plaster  walls and many cast objects. Gypsum has a very low thermal conductivity,  so it is an excellent, low cost, insulating material for buildings. The  water within gypsum’s crystalline structure also actually helps to  minimize fire damage. If a fire breaks out, heat drives the water out of  the gypsum walls to cool and protect the wood or steel supporting the  walls. Consequently, one of the main modern uses of gypsum is the  manufacture of plasterboard (also called drywall or sheetrock) commonly  used in construction. The average American home may contain as much as  seven tons of gypsum, this mass of plaster translates into one and a  half tons, or roughly 350 gallons, of water built into your home’s  structure. While this is not enough to quench a major fire, it is enough  to reduce the damage of a small blaze.
The ease with which gypsum loses and regains its water of  crystallization also makes it ideally suited for sculpture and casts.  Gypsum has many applications in art and pottery, but is also used in  medicine as casts for broken bones or as dental molds for making  artificial teeth. Its many other uses include the ‘paste’ component of  toothpaste, modern chalk used in classrooms, and as filler for paper and  paints. It is also an important component added to cement, to slow the  rate at which concrete settles or hardens. Large amounts of gypsum are  used as fertilizer or as a conditioner for soil, displacing sodium in  the soil and allowing the soil to hold more moisture.
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Gypsum in the Upper Midwest:
Few concentrated gypsum deposits occur in the Upper Midwest, but  gypsum crystals, precipitated from groundwater, are disseminated through  some local carbonate rocks and can be found in old mine shafts in  Wisconsin. Selenite gypsum crystals also occur in Cretaceous-aged clays  in central and western Minnesota.