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- Jacinth an unsatisfactory name variously applied to the red zircon and to the similarly coloured hessonite garnet.
- Jade a term used for both jadeite and nephrite (which see). Other massive material sometimes confused with the jades are as follows:
- (1) Californite; green compact idocrase. Grossular Garnet;
- (2) Serpentine
- (a)Antigorite (thin and platy).
- (3) Hornblende;
- (4) Quartz; Chalcedony (Chrysoprase and Plasma).
- (5) Saussurite,
- (6) Smithsonite;
- (7) PreLnite,
- Jadeite ("Jade" part) R.I. 1-65 to 1-68; S.G. 3-3 to 3-5; H. 6.5 to 7; Monoclinic; Colours, white, green, pink, lilac, mauve and brown; Locality, Burma. See also Chloromelanite.
- Jade matrix (Jade albite) a rock consisting of a mixture of nephrite and albite. Sometimes called "snowflake jade".
- Jam-Peg an upright post, positioned at the side of the polishing lap and containing a number of suitably placed holes into which is placed the end of the gem holder, thus regulating the angle of the facet being cut. See Gem stick.
- "Japanese pearls" cultured blister pearls finished with a flat base.
- "Japan pearls" cultured blister pearls finished with a spherical base. Sometimes called Half-cultured pearls.
- Jargoon outmoded name for pale coloured zircons.
- Jasper an impure form of compact quartz.
- Jet a form of fossil wood allied to cannel coal; R.I. 1-64 to 1-68; S.G. 1-10 to 1-40; H. 3.5; Localities, Spain, and Yorkshire (England).
- Jolly's spring balance a balance consisting of a spring suspended vertically against a graduated scale. At the lower end of the spring is attached two scale pans one above the other, the lower pan being immersed in water. The density is determined by recording the reading on the scale of a pointer situated at the lower end of the spring when there is no specimen in either of the pans (reading A), when the specimen is in the top pan, that is in air (reading B), and when specimen is in the lower pan, that is in water (reading C). Formula: B minus A divided by B minus C gives the specific gravity.
- "Jourado diamond" a name, reported from the U.S.A., as being an incorrect term for an imitation colourless stone. The term may have had its origin from the so-called "diamond scare" of 1935 in England, in which case the material involved was synthetic colourless spinel. The term is definitely a MISNOMER ( COMMERCIAL LABEL ).