The dopants used to impart color in artificial simulants may be detectable as a complex rare-earth absorption spectrum, which is never seen in diamond. Also present in most diamonds are certain internal and external flaws or "inclusions", the most common of which are fractures and solid foreign crystals. Artificial simulants are usually internally flawless, and any flaws that are present are characteristic of the manufacturing process. The inclusions seen in natural simulants will often be unlike those ever seen in diamond, most notably liquid "feather" inclusions. The diamond cutting process will often leave portions of the original crystal's surface intact. These are termed "naturals" and are usually on the girdle of the stone; they take the form of triangular, rectangular, or square pits ("etch marks") and are seen only in diamond. Diamond is an extremely effective thermal conductor and usually an electrical insulator. The former property is widely exploited in the use of an electronic "thermal probe" to separate diamonds from their imitations. These probes consist of a pair of battery-powered thermistors mounted in a fine copper tip. One thermistor functions as a heating device while the other measures the temperature of the copper tip: if the stone being tested is a diamond, it will conduct the tip's thermal energy rapidly enough to produce a measurable temperature drop. As most simulants are thermal insulators, the thermistor's heat will not be conducted. This test takes about 2–3 seconds. The only possible exception is moissanite, which has a thermal conductivity similar to diamond: older probes can be fooled by moissanite, but newer thermal and electrical conductivity testers are sophisticated enough to differentiate the two materials. The latest development is nano diamond coating, an extremely thin layer of diamond material. If not tested properly it may show the same characteristics as a diamond. A diamond's electrical conductance is only relevant to blue or gray-blue stones, because the interstitial boron responsible for their color also makes them semiconductors. Thus, a suspected blue diamond can be affirmed if it completes an electric circuit successfully. Diamond has been imitated by artificial materials for hundreds of years; advances in technology have seen the development of increasingly better simulants with properties ever nearer those of diamond. Although most of these simulants were characteristic of a certain time period, their large production volumes ensured that all continue to be encountered with varying frequency in jewelry of the present. Nearly all were first conceived for intended use in high technology, such as active laser mediums, varistors, and bubble memory.