• man-made substances are not usually considered mineral, they are compounds.
• substances produced biologically are not usually considered to be minerals; however, certain minerals can be produced or precipitated by biologic processes.
• the mineral must have a repeatable crystal order. This leads to the concept of symmetry. Symmetry is the systematic repetition of identical features. You can think of symmetry when you're tiling a floor, only regularly shaped, repeatable patterns can completely fill the space. Possible symmetries are:
 
1-fold
like a dot on a plane

2-fold
like a line in a plane
2 mirror planes

3-fold
like a triangle on a plane
3 mirror planes

4-fold
like a square on a plane
4 mirror planes

6-fold
like a hexagon
6 mirror planes

5-fold (quasi-crystals) - not natural

Crystal form - one or more sets of symmetrically equivalent faces on a crystal.

Crystal habit - general shape of the crystal such as:

acicular: long, needle-like crystals
prismatic: thicker than acicular, similar to columnar
equant: stubby, equidimensional crystals
tabular: crystals that occur as flattened planes
micaceous: extremely flattened into flexible sheets
bladed: flat and elongated like a knife blade
blocky: equant crystals with square
filiform: hair-like or thread-like
dendritic: in slender divergent branches
drusy: dusty, or sugary, coating of crystals
botryoidal: globular form, like a bunch of grapes
reniform: kidney-shaped

Cleavage: mineral breaks along definite planar surfaces; parallel to crystal face; should note the number of cleavage directions, their angles, and their perfection
Parting: mineral breaks along a structural weakness; may be because of twinning (not cleavage)
Fracture: how a mineral breaks when it's not cleaved or parted, e.g.:

conchoidal: smooth curved surface; like a shell
fibrous or splintery
hackly: jagged fractures with sharp edges
uneven or irregular: rough surface

Links on the Mohs Hardness Scale are to the Amethyst Galleries' Mineral Gallery; a nice place to browse a whole assortment of minerals.

Mohs hardness scale

Luster: the appearance of the mineral in reflected light; either metallic or non-metallic. Terms used to describe luster:

vitreous: glass-like
resinous: appearance of resin
dull: earthy appearance
pearly: milky reflectance; iridescent
greasy: oily appearance
silky: fibrous or silky appearance
adamantine: hard, brilliant luster

Color: note color and if there are any differences between fresh and weathered surface; also note if the mineral changes color when moved; may have an opalescence or iridescence.

Streak: color of the mineral powder on a white streak-plate.

Diaphaneity: ability to transmit light

transparent: can see through the mineral
translucent: light is transmitted near the edges
opaque: no light is transmitted

Tenacity: resistance of mineral to breaking.

brittle: breaks or powders easily
malleable: can be hammered into thin sheets
sectile: can be cut into shavings with a knife
ductile: can be drawn into wire
flexible: bends but does not break
elastic: after being bent will return to original shape.

Specific gravity: relative density of the mineral. Ratio of the mineral to an equal volume of water at 4°C.
 
 

Other physical properties:

magnetism
luminescence (thermo, tribo)
taste
fluorescence (phosphorescence)
odor
piezoelectricity
feel
pyroelectricity
refraction
solubility in HCl (effervescence)