1 Scope
This standard specifies the terms, testing methods and testing criteria of gems.
This standard is applicable to the determination of gem varieties.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute provisions of this standard. For dated references, subsequent amendments (excluding corrections), or revisions, of any of these publications do not apply to this standard. However parties to agreements based on this standard are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition applies.
GB/T 16552 Gems - Nomenclature
3 Terms
For the purposes of this standard, the following terms apply.
3.1
crystal
solid with a lattice structure, of which, the internal particles are regularly arranged in a regular periodic arrangement in space
3.2
crystalline
crystalline solid (crystal)
3.3
crystalline aggregate
block composed of numerous crystallines, which includes phanerocrystalline aggregate and cryptocrystalline aggregate
3.4
non-crystalline
solid of non-lattice structure, of which, the internal particles are irregularly arranged in space
3.5
crystal system
crystal systems are classified as follows: cubic system, hexagonal system, tetragonal system, trigonal system, orthorhombic system, mono-clinic system and triclinic system
3.6
crystal habit
habit of certain mineral of tending to crystallize into a certain form under certain external conditions
3.7
twin, twinning striation
twin refers to the regular formation of two or more of the same crystals according to certain symmetry rule; it is classified into contact twin, interpenetrant twin and ring twin according to the individual formation mode of twin. The contact twin is further classified into simple contact twin and polysynthetic twin
twinning striation refers to the linear striation appearing on the crystal face, cleavage plane or gemstone cutting plane of twin joint surface
3.8
crystal face, striation
crystal face refers to the plane encircling the crystal surface, which is naturally formed during crystal growth
striation refers to the straight striation of the same simple form on a crystal face, also called growth striation
3.9
colour
feeling that produced in the brain due to the sense of optical wave (390~780nm for visible light) by the fundus optic nerve; the colour produced by the mixing of the remaining light waves of the visible light after being selectively absorbed by an object is the colour of the object
3.10
colour band
uneven distribution of internal colour in a band (or block) shape; the original colour band is a change in colour depth or colour due to changes in medium composition and growth environment during crystal growth. For example: sapphire, tourmaline (taltalite)
3.11
optical character
phenomena generated due to the action of material in the incidence and propagation directions of light, including such characters as the isotropy and anisotropy of material, axiality and positive/negative characters of anisotropic material
3.12
isotropic material
material with optical character the same in all directions; the cubic-system and non-crystalline materials are isotropic
3.13
anisotropic material
material with optical character different in each direction; all materials other than cubic-system and non-crystalline ones are anisotropic
3.14
uniaxial crystal
crystal with only one special direction (one optical axis), which is free of birefringence when the incident light is parallel to this direction; the crystals of trigonal system, tetragonal system and hexagonal system are uniaxial
3.15
biaxial crystal
crystal with two special directions (two optical axes), which is free of birefringence when the incident light is parallel to the two directions; the crystals of orthorhombic system, mono-clinic system and triclinic system are biaxial
3.16
positive character, negative character
the uniaxial crystal gemstone is of positive character when its ordinary light refractive index is less than the maximum extraordinary light refractive index, otherwise, it is of negative character
3.17
refractive index, birefringence
refractive index is the ratio of the propagation speed of light in air (or vacuum) to that in gemstone material
birefringence is the maximum difference between two or three main refractive indexes in anisotropic material, also called double refraction
