Tabular
It is term for the crystal habit which describes a flat, tough and usually four sided crystal.

Tarnish
It is the property in some minerals which causes them discolor if placed in certain environments. Silver is a good example that develops a yellowish-black coating on its surface after being exposed to hydrogen sulfide (H₂S). In the atmosphere, H₂S is present in small quantity.

Tectosilicates
It is a group of silicate minerals having all the four oxygen atoms from each tetrahedron sharing itself with an adjacent tetrahedron.

Telluride
It is a mineral which is formed by the combination of tellurium with another chemical like calaverite (AuTe₂).

Tenacity
It is the reaction of a mineral to certain stress.

Termination
It is the end of a crystal face, generally referring to its base.

Tetragonal (crystal system)
Any mineral which has three axes, two of them equal in length and one unequal and all the three axis are at 90° to each other.

Tetrahedron
It is four sided polyhedron. All the sides are equidimensional and triangular shaped. Members belonging to it belongs to the isometric system. Also known as Tetrahedra or Tetrahedrons in plural form.

Texture
It is the feel and appearance of a mineral, like color, crystal form, and luster.

Transparent
It is term used to describe an object which has the ability to transmit light through it. If any other object is placed in front of it, that object can be seen, as if there is nothing interfering.

Translucent
It is a term used to describe an object which transmits light, but partly. Objects placed in front of it will be seen, but not in clear form.

Triassic
It is a period of geological time which extends from 250 to 205.1 million years ago. It marks the begining of the Mesozoic Era.

Tungstates
It is the group of minerals which are compounds of one or more metallic elements and the tungstate radical (WO₄). They are very heavy, soft, and brittle. Tungstates are generally categorized with the chemically related molybdates, where they can be partially replaced by. They are also known as the wolframates.

Twinning
It is the tendency of some crystal to intergrow in a distinct specific form and repeated patterns.