The cannabis industry frequently makes use of chromatography, which is a technique for separating and purifying cannabinoids, in order to separate THC and CBD from cannabis that has already been extracted.
The vast majority of people who use cannabis are not going to have access to the commercial chromatography equipment that is required to generate crystals, but as the following video demonstrates, there are ways to create a chromatography machine on a smaller scale. This method requires a collection chamber that has a small amount of an evaporable solution inside of it, a pump to apply pressure (the pump should have required tubing and a lid that fits the column), and a column that has fine sand inside of it (silica is a popular agent that is used amongst cannabis producers). The method that is demonstrated in this video can be found here.
The unpurified extract is introduced to the column, which then functions as a filtering system due to the presence of the sand. The application of pressure causes the molecules to be forced through the sand, resulting in a greater degree of purification. The process will still be effective even if there is no pressure available; however, it will take a great deal more time to complete. When the molecules move through the sand, the more heavy molecules (THC) move more slowly, if at all, while the smaller particles (CBD) move to the bottom of the container more quickly.
The filtered extract is then collected and kept in the solution reservoir once it has been filtered. This process can be carried out multiple times to achieve the highest possible level of purity; however, it is important to keep in mind that the quantity of the final product will decrease proportionately with each additional cycle. Following the completion of the filtration process, the solution should be placed on a heat source, like as a coffee pad, to cause the solution to evaporate. This will result in the production of CBD crystals that have been cleaned.