AR (anti-reflective) coating is a free option that is available and very popular. It is available in a choice of standard colours/tints with blue being by far the most popular choice. As well as having anti-reflective properties a coloured AR coating is very popular with watch modders by enhancing the finished creation. The coloured tint of the AR coating will only reveal itself in certain light conditions/angles. The coating is applied on the underside only. There is nothing quite like double domed sapphire crystal with AR coating to lift what is a great mod to a fantastic one. Currently out of 100 sapphires sold 95 will be with blue AR. AR coatings in addition to the standard range of blue purple green may be available at extra cost and additional lead time please inquire. Sapphires with no AR coating are always available these are particularly popular for the classic vintage watches.
An AR coating reduces light reflection at a glass surface from approximately 4.1% to anywhere from 2% to ~0.1% over the visible wavelength range (400nm-700nm) depending on how well it is made. So technically a watch with no coatings should reflect ~8% of the incoming light and the one with the single side coating should reflect less; somewhere in the 4-6% range. Probably closer to 4.5% as a <0.5% spec is common.