What mineral is this?

People who ask these questions are called mineralogists. They use a specimen's physical properties to decide what mineral they are dealing with. These properties are:

Crystal Form - arrangement of the crystal surfaces. This is relates to the conditions at the time of formation and knowledge of how atoms tend to link together. Examples are cubes and needle-like crystal structure.
Color - a poor indicator but will show if impurities are present.
Streak - the color of a mineral in powered form rubbed on a non glazed porcelain plate.
Luster - the quality, appearance and intensity of light reflected from a mineral. Metallic, pearly, greasy, etc.
Cleavage - the tendency of a mineral to break in preferred directions ... leaving flat smooth surfaces.
Hardness - the resistance to abrasion ... performed using a scratch test using one mineral against another. The Mohs Scale lists the 10 standards from diamond = 10 (hardest) to talc = 1 (softest). Talcum power is pulverized talc (also known as soapstone).
Specific Gravity - same as density (how compact the matter is).
Texture - size and arrangement of interlocking crystals in an igneous rock
Taste & Feel - we often add sodium chloride (NaCl -table salt) to our food. Other halides also have unique tastes.
Tenacity - resistance to bending, breaking, and other forces
Magnetism - some minerals are simply attracted to magnets, others can become permanent magnets.
Effervescence - bubbles in the presence of HCl (hydrochloric acid) A test for calcite
Electrical Conductivity - Metals are good electrical conductors, non-metals are good insulators.
In addition, other tests involve polarizing microscopes, X-rays, and spectral analysis.