Recently we were asked whether fluorite is a good ringstone.
The answer is NO! Not at all!
We take the opportunity to compile a (incomplete) list of gems unsuitable or restrictedly suitable for jewellery use.
By the way: jewellery TV channels do offer most of these gem stones set in rings...
Anglesite: hardness 2.5-3, perfect cleavage, sensitive to heat, acids and pressure, scratches easily and thus is unsuitable for jewellery use
Apatite: hardness 5, no distinct cleavage but very brittle and sensitive to pressure. Very sensitive to heat and acids. Do not clean ultrasonically! Because of the low hardness and the sensitivity to shock and pressure apatite is not recommended for use as a ringstone
Aragonite: aragonite is soft and brittle, has good cleavage and is quite sensitive to acid. Use in jewellery not recommended.
Cassiterit: hardness 6-7, unfortunately very sensitive to acids and heat, very brittle
Cyanite: due to low hardness and perfect cleavage, cyanite is only of limited use as a gemstone. Besides cyanite is very sensitive to heat and should neither be cleaned ultrasonically, nor should it be exposed to galvanic treatments. Cyanite sports so-called anisotropy of Mohs hardness, which can be as low as 4.5 along the c-axis and as high as 7 along other crystallographic axes.
This, as well as the perfect cleavage, makes cyanite rather difficult to cut and polish.
Fluorite: Mohs hardness 4, perfect cleavage in three directions, highly sensitive to acids
Haüyn: has perfect cleavage and is rather brittle. On top of that it is also very sensitive to heat and acids. Do not clean ultrasonically!
Hiddenite: reasonably hard (6.5-7) but perfectly cleavable and very sensitive to pressure and heat. Not a good ringstone at all!
Kunzite: same as hiddenite: hard but perfectly cleavable and very sensitive to pressure and heat.
Scheelite: sensitive to acids and pressure, scratches easily, unsuitable for jewellery use
Scapolite: Mohs hardness 5.5-6, sensitive to heat and acid, perfect cleavage – limited use in jewellery, not recommended as ringstone
Sphalerite: soft (hardness 3.5-4) and very sensitive to heat, acids and brines. On top of that is has perfect cleavage in six directions. All this renders sphalerite unsuitable for use in jewellery
Sphene: due to it´s relatively low hardness, distinct cleavage and sensitivity to acid, brine and heat, suitability as a gem stone is limited. Do not clean ultrasonically, no galvanic baths, shock-sensitive. Handle with great care.
Wulfenite: because of low hardness (2.5-3), good cleavability, sensitivity to heat and brittleness, wulfenite is unsuitable for jewellery use.
Deutsch