The most widely used substitute for lapis for many years has been blue-dyed chalcedony. The main cause for confusion is not that it bears a close resemblance to lapis, but that the average jeweler is accustomed to seeing it and hearing it referred to by the trade names "German lapis" and "Swiss lapis". Actually, both the color and general appearance of dyed chalcedony differ markedly from lapis. Its fracture surfaces of lapis. However, the luster of its fractures is dull to waxy, not appreciably different than the luster of rough lapis. Usually, since chalcedony is well polished, an accurate R. I. reading can be taken; this permits a positive identification, because the reading is distinctly higher 0.53 compared to 1.50 than that of lapis. (Note: Because most specimens of lapis are poorly polished, it is difficult to determine R.I. when the polish is sufficiently good, it must be embarked that lapis is a rock and that the R.I. of the pyrite or calcite portions will be entirely different from that of the blue portion. Thus, care must be exercised to make certain that only the blue part is being tested ). A streak test is also valuable, since the streak of lapis is usually pale blue and chalcedony cuts into the testing plate, rather than leaving a streak.
Glass rarely has the same appearance as lapis; when it does however, it can be detected by swirl marks, mold marks, and especially gas bubbles, which are present in profusion and usually detectable under magnification near the surface. Moreover, the conchoidal, various fractures of glass are completely unlike the uneven, dull, granular fractures of lapis.
Opaque synthetic spinel is made that resembles good-quality lapis. Some of the stones made from this material even have metallic inclusions, which are produced by forcing pure gold into holes in the surface. These stones have a much higher luster than lapis and are usually well polished; thus, an R.I. determination will usually ascertain identify immediately. If doubt exists, the metal particles can be tested with a needle. The hard pyrite inclusions in the genuine material are difficult to scratch, but the gold particles in synthetic spinel are very soft by comparison and can be scratched with no effort.
There are three natural gem minerals that may resemble lapis very closely: azurite, lazulite and sodalite. If these stones are well polished and an R. I. can be obtained, both azurite and lazulite can be separated easily because of their much higher indices. However, the R.I. of sodalite is similar to that of lapis. If the index cannot be determined, or if sodalite is suspected, a separation can be effected by S.G. for both azurite and lazulite are noticeably higher than lapis and the value for sodalite is definitely lower. The difference in S.G. is particularly noticeable, since confusion would normally occur only with finer specimens of Lapis without pyrite inclusions. The presence of pyrite practically eliminates azurite and lazulite. Sodalite, a member of the same group of minerals as lapis, almost never contains pyrite, so this is of value in separation.
However, it is possible for sodalite to be one of the constituents in the rock that is lapis; as a practical matter, therefore, they are not far from being identical. S.G. does provide an effective means of separation, however, another less frequently encountered gem mineral that may bear a marked resemblance to lapis is the massive, violet-blue pyroxene known as violane. It can be distinguished by its higher R. I. (1.69) and S.G. (3.23)
Since lapis-lazuli is an opaque material in gem sizes, polariscope tests are obviously of no value. If acid tests are made, only a very small drop should be applied to the stack of a stone, in order to avoid damage. Afterward, it should be washed thoroughly.
