To test a rock for gold, first perform nondestructive visual inspections and tests for weight and magnetism. You can also use destructive methods such as testing hardness, streak, ductility, or pulverizing and panning the powder. You will need a scale, glass, unglazed ceramic, and a powerful magnet.

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 — Quartz. Quartz is classically aligned with gold. The majority of hard rock deposits found appear to occur in quartz, and gold-in-quartz has even taken on a life of its own. The gold that appears in quartz is almost universally native, meaning that it's in a metallic form contained in the crystalline quartz.

Finding Gold in Mexico: Search carefully for the presence of quartz veins, limonite, calcite and adularia, and rocks with textural and mineralogical evidence for the past presence of metalliferous fluids. Where these elements are abundant, gold is frequently close by.

 — Discover the fascinating world of rocks and minerals containing gold, from quartz to calaverite, and their significance in gold mining.

 — The state of Zacatecas has certainly produced superb collector minerals including azurite, galena, sphalerite, chalcopyrite and other metal ores. And, of course, silver species and gold have also come from here. San Luis Potosi is very well known among collectors due to the superb poker chip calcite specimens it yielded in recent years.

 — The gold in your piece is most likely in the 20K range or around 83.3%, but some places like Australia regularly have higher purities in their hard rock ore. Impurities can include silver and copper, but lead is another common one.

 — Quartz veins are closely associated with the presence of gold and are visible in rock faces and cliffs in the surrounding area. Weathered chunks of quartz lying on the ground and smaller quartz stones and pebbles in rivers and streams or …

The coarse and easily visible gold in quartz may be valuable as a specimen or for making gold-quartz gemstone jewelry. Knowing just how much gold is in your quartz is important, and I've written a sidebar article on estimating the gold in quartz (see "Determining the Amount of Gold in Rich Ores").

 — Quartz gold ore is found in large veins of quartz, most commonly rose quartz and rainbow quartz. If the gold is on the surface of the rock it'll look like shiny yellow flecks or veins of gold. Iron oxide copper gold ore is found on the edges of granite or as long bands of dark rock flecked with gold.