Cutting rocks creates friction and heat. Harder and larger rock take longer to saw through, creating more heat and friction. Lubrication of some kind reduces the friction and keeps the rock from shattering and the blade from becoming too hot. While rock cutters previously used kerosene or diesel oil, the smell, mess and flammability of these fluids make them undesirable lubricants today. Rock saw users do not agree on one universal oil, but give reasons for their preference of one oil over another.
Water-soluble coolants are recommended by some gem cutters, but only for smaller and more porous stones, such as marble, granite, turquoise and travertine that might absorb oil into the stone if an oil-based product were used. The advantage to these products is they are not as messy as oil and don't build up as much sludge. If you use a water-based coolant, be sure that it has rust and corrosion inhibitors. Even with rust inhibitors, it is advisable to empty your saw of the water-based coolant every day, wipe it dry and spray it with WD-40 oil so rust does not occur.
Saws that are 10 inches or larger work best with petroleum-based lubricants. They usually are pre-mixed and do not require diluting. Oil lubricants are needed for hard stones, such as quartz, jasper, agate and petrified wood. Check to be sure that the oil can be left in your saw without damaging it. The disadvantage to oil lubricants is that they are messy and build up layers of dirt, rock dust and oil sludge more than water-based lubricants do. Make sure that the one you choose does not have the strong odor that earlier oils did.
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Although not recommended by lapidary experts or most rock saw users, some users do use mineral oil or baby oil as a rock saw lubricant and would not use anything else. They like mineral oil because it is inexpensive -- often found at two-for-one sales and at dollar or discount stores. It has a pleasant smell when first used, but this goes away with the high heat of the saw, leaving virtually no odor at all. Mineral oil does not spoil the way cooking oil can, which is why using cooking oil is not recommended.
Oil used in rock saws can be left in the saw because they will not cause rust the way water-based lubricant can. Eventually, though, the rock dust and dirt and oil will make a thick sludge as you use the saw. On top of the sludge is cleaner oil that can be reused. If you strain the sludge through two paper bags, put one inside the other, the sludge will stay in the bag while the reusable oil will drain through. You will save money over buying new oil and the time and money of a trip to get it.