Treatment: Treatment in gemology means applying any method that makes the appearance of a gemstone more attractive (at least the person that makes the treatment thinks so).
There are many methods of treatment that oil treatment,heat treatment and dying are examples.
Note that cutting and polishing of gemstones are not treatment.
Botryoidal or botryoid: A formation sometimes seen on surface of some stones and gemstones that resembles a cluster of grape. Botryoidal hematite is an excellent example.
Color zoning: Color zoning means uneven distribution of a certain color inside a crystal. Ruby, sapphire, citrine and amethyst are famous for exhibiting color zoning, though not above mentioned stones are necessarily supposed to have color zoning always.
Pleochroism: Pleochroism is a special phenomenon that occurs when a crystal is viewed from different angles or direction and causes a significant change in color. A very good example is iolite that can be seen violet blue and then changes to grey when the direction of view is changed.
Color change: Color change is a phenomenon like Pleochroism that causes color changes in crystal, but there is a very important difference. Color change phenomenon occurs when lights of different color (different wavelength) are shined on a crystal. Alexandrite is very famous for color change effect and some other stones like sapphire sometimes exhibit color change effect.
PVD: PVD (Physical Vapor Deposition) is a kind of treatment. During PVD process vapor of a metal is deposited on a crystal creating a very thin film of the related metal. This kind of treatment usually creates special colors and even rainbow phenomenon on a gemstone.
Clear quartz (also called white quartz or rock crystal) and topaz are the stones that are often used for PVD process.
PVD is used extensively because of the excellent durability of deposited metal compared with electrolytic deposition of metals and the fact that virtually all subject can be covered with metal using this process, unlike electrolysis that is only applicable when the surface of the object can conduct electricity.
Mohs hardness: Mohs hardness is a scale used to provide an understanding about hardness of substances. Mohs scale uses numbers, for example the hardest natural substance is diamond with a Mohs number of 10.
Mohs hardness scale is not an absolute measurement method and is relative, that means it provides a method for comparing hardness and scratch resistance of substances.
Mohs scale is used extensively in gemology and industry.
Luster: Luster is the appearance or look of the surface of a stone or crystal.
Luster of a gemstone depends on how it reflects the light that hits the surface and each gemstone has a specific luster because of chemical composition and molecular structure of related stone.
There are many types of luster like wax like, vitreous (glassy) and even wet glassy (wet vitreous look).
Geode: Geodes are sphere or ball like formations with hollow inside most of the time. Inside of the geodes are covered with crystals and sometimes the crystals are so dense that there is actually not a hollow space.
A geode might look a worthless round stone to someone that does not know what it really is.
Geodes are cut or broken open to exhibit crystals.