The gems that are similar to crystals are mainly fluorite, calcite, feldspar, tourmaline, aquamarine, sageite, etc. The similarity is mainly reflected in appearance characteristics such as color and transparency. However, there are many differences between these gems and crystals.
Amethyst and purple fluorite
The chemical composition of fluorite is calcium fluoride, which is usually cubic crystal form. Mohs hardness is 4, there are four groups of obvious cleavage, homogeneous body, totally dark under polarized lens, refractive index of 1.434, density of 3.18 g/cm3, no pleochroism, step-like fracture. The crystal is a combination of hexagonal column and rhombohedron. There are horizontal stripes on the cylindrical surface. The Mohs hardness is 7, without cleavage. There are four bright and four dark phenomena when rotated 360 degrees under the polarizer. The refractive index is 1.544~1.553, and the density is 2.64 grams per cubic centimeter. It is dichroic and has a shell-shaped fracture. In the rough crystal trading market, people often use purple fluorite crystal clusters as amethyst for sale, and the asking price is higher.
Crystal and calcite
The chemical composition of calcite is calcium carbonate, the crystal form is rhombohedron, the Mohs hardness is 3, there is obvious cleavage, the refractive index is 1.48 to 1.66, the density is 2.7 grams per cubic centimeter, and the stepped fracture. Crystal is a hexagonal column and rhombohedral polytype, with horizontal stripes on the cylindrical surface, Mohs hardness of 7, no cleavage, shell-shaped fracture, refractive index of 1.544 to 1.553, density of 2.64 grams per cubic centimeter. Calcite imitation crystal balls are common in the market. During identification, you can see whether there is cleavage or scratch with a knife. The crystal can’t be scratched by the knife, but the calcite can be scratched by the knife. Hydrochloric acid can be dripped to identify the rough stone, the calcite bubbling after adding hydrochloric acid, the crystal has no reaction.
Citrine and Yellow Topaz
Yellow topaz is a kind of silicate mineral. It has stronger luster than crystal. It has strong glass luster. It is transparent and flawless. Its density is 3.53 grams per cubic centimeter. The crystal shape is columnar. The cylindrical surface often has longitudinal lines and has a group of Completely cleavage, the refractive index is 1.619 ~ 1.627, there are liquid inclusions that are not miscible with each other, and the shape is a droplet shape. The density of the crystal is 2.64 grams per cubic centimeter, without cleavage, and the refractive index is 1.544 to 1.553, which is easy to distinguish. The easiest way is to weigh stones of the same size by hand, and the yellow topaz feels heavier obviously.
Colorless crystal and colorless feldspar
Feldspar is a silicate mineral, mostly plate and short columnar crystals. Both colorless feldspar and colorless crystal are colorless and transparent crystals, and their refractive index and density are similar. The best way to distinguish is to observe the inclusions inside the gem. Feldspar develops two sets of nearly vertical cleavage, so it often appears as a “centipede” inclusion inside the gem; the fracture is a stepped fracture. Crystals are cotton-like inclusions and needle-like inclusions, with a conchoidal fracture.
Colorless crystal and light-colored tourmaline
Tourmaline is a silicate mineral with columnar crystals with longitudinal stripes on the crystal surface. The cross-section of the column is spherical triangle, with a refractive index of 1.624 to 1.644, a density of 3.06 grams per cubic centimeter, and a stable birefringence of 0.02 . It is a bit difficult to distinguish the finished product with the naked eye, but it is easy to distinguish between crystal and tourmaline using instrumental testing.
Colorless crystal and aquamarine
Aquamarine is a kind of silicate mineral. Its crystal form is hexagonal columnar. There are longitudinal lines on the crystal surface. Both of them have developed cracks. When observed with a magnifying glass, the inclusions inside the aquamarine are “rain-like” with a refractive index of 1.577-1.583 , The density is 2.72 grams per cubic centimeter, which is obviously different from crystal.
Crystal and Sugilite
Sugilite is a deep purple gemstone, often appearing in aggregate state, with poor transparency. The refractive index is 1.16, and the density is 2.74 grams per cubic centimeter. Because it often contains quartz impurities, the measured refractive index is 1.54, which is consistent with crystals. Pay attention to several points.
Amethyst, Citrine, and Scapolite
The colors of the two are similar, and the refractive index ranges are similar. If you don’t pay attention, you will get confused. The scapolite is a tetragonal columnar crystal with longitudinal lines on the cylindrical surface. The color distribution of crystal is often uneven, with color bands and color blocks, and the color of scapolite is relatively uniform. The inclusions of scapolite are straight and thin tubes, and the inclusions of amethyst are irregular gas-liquid shapes. The scapolite has a set of medium cleavage, and the purple crystal has no cleavage. Under the polarizer, the crystal is a bull’s eye-shaped interferogram, and the scapolite is a cross-shaped interferogram.
Amethyst and Cordierite
Although the refractive indexes of the two are similar, they can be distinguished with a little attention. The main difference is: the color of cordierite is purple-blue to blue, and there is rarely the purple of amethyst. Cordierite has obvious dichroism, and it can be seen without a dichroic mirror, and the dichroism of amethyst is not obvious.