Article: Carbide scribes are tipped with tungsten carbide, a high-quality cutting material that's used in some of the best glass cutters. For cutting glass without a glass cutter, a scribe will be your best option. These tools don't have a cutting wheel, but the tip of the scribe is extremely fine and can easily score glass. You should be able to purchase a carbide scribe at most large hardware stores or home-improvement stores. It costs less than $10 USD. Diamond scribes are used to score sheets of metal for cutting in industrial settings. As such, they're more than sharp enough to score a sheet of mirror glass. They'll also last longer than most carbide scribes.  You can purchase a diamond scribe through large online retailers or at some craft-supply stores. Diamond tipped scribes are about twice as expensive as the carbide tipped option. They'll cost about $20 USD. Inexpensive glass cutters have steel blades, so a steel file works as an approximate replacement. If you don't have a steel file, you can purchase one at a hardware store for under $10 USD. The file doesn't need to be large, but should have relatively sharp, angled edges to cut the glass. To cut glass with steel, you'll position the sharp edge (or pointed tip) of the file against the mirror, then scrape it back and forth until you've lightly scored the glass. Be aware that using a rough steel edge (like a file) to cut glass may result in an imprecise cut and a sloppy break. A carbide scribe will produce a sharper, more clean and accurate cut.

What is a summary?
Pick a carbide scribe for a sharp, precise cut. Select a diamond-tipped scribe for a longer-lasting tool. Use an ordinary steel file for a cheap option.