The 1870-1871 Franco-Prussian War Medal (Kriegsdenkmünze für 1870/1, abbreviated KDM) was awarded to Prussian forces and allies for the war against France.  There were 2 different ribbons authorized:  one for those who served in France and one for those who served but not in France, irregardless of if they were awarded the combatant or non-combatant medal. The award certificate and/or Soldier's Book makes mention of which ribbon was earned.

The ribbon for service in France is exactly the same as for the Iron Cross.  It is black with 2 white stripes near each end and with an additional central red stripe - the same width as the white stripe. 

(photo pending)

The ribbon for service (but not within France) is exactly the same as for the non-combatant (Civil) Iron Cross.   It is black with 2 white stripes near each end and with an additional central red stripe - the same width as the white stripe.

(photo pending)

The original ribbon was a little over 3cm wide (3.01cm to 3.09cm - depending on the weaver) and had a fine silk weave with an intricate weft design.  Replacement ribbons can be made of most any material - with a poly based weave and minimal weft design being the most common current production, making it almost twice as thick as the original silk ribbon!  Since the Franco-Prussian War Medal  and the WWI Cross of Honor have the same ribbon colors, the following notes provide a brief guide on the differences.  These are only typical examples used for simplicity of explanation and is not an exhaustive list.

• Contemporary Franco-Prussian War Medal  Ribbon:  3.0cm wide, but often found at ca. 3.02cm (if used - the material stretches with age). The weft is often of a slightly wavy appearance and is rarely straight. The horizontal weave is most likely to have a manufacturing defect which may be visible under a magnifying glass. The physical texture is different to the experienced touch and is difficult to explain in a guide. Silk, the weave is 16-17 rows per cm, each thread is 2 strands of ca. 12 fibers per strand, and it is UV-negative.  Silk, the weave is 16-17 rows per cm, each thread is 2 strands of ca. 10 fibers per strand, and it is UV-negative. These ribbons are rarely available on the market in unused condition.
• Contemporary WWI Cross of Honor Ribbon:  Often used as a replacement ribbon for the Franco-Prussian War Medal since the width and colors are the same - almost all collectors make no distinction in the ribbon differences. Silk, the weave is 15-16 rows per cm, each thread is 2 strands of ca. 20 fibers per strand, and it is UV-negative.  Silk, the weave is 15-16 rows per cm, each thread is 2 strands of ca. 15 fibers per strand, and it is UV-negative. Experience can help the collector differentiate between the ribbons although there are a few cases where a complete analysis might be required. There are a few cases where the results may prove inconclusive but the majority can be determined. These ribbons can sometimes be found on the market in unused contion, but not very often.
• WWII-era Replacement Ribbon: Silk, the weave is 14-15 rows per cm, each thread is 2 strands of ca. 15 fibers per strand, and it is UV-negative.  Cotton, the weave is 14-15 rows per cm, each thread is 2 strands of ca. 10-12 fibers per strand, and it is UV-negative. It's important to note that by the time these replacement ribbons were introduced, most of the recipients of the Franco-Prussian Medal had died. These ribbons are often found on the market and are routinely offered as "old" ribbons and often imply that these were original ribbon lengths of the Franco-Prussian War Medal and/or WWI Cross of Honor.
• Modern-era Replacement Ribbon: Often found in widths of exactly 3.00cm or 3.10cm.  Silk, the weave is 14 rows per cm, each thread is 3 strands of ca. 10 fibers per strand, and it is UV-negative.  Cotton, the weave is 13 rows per cm, each thread is 2 strands of ca. 15 fibers per strand, and it is UV-negative.  Poly, the thread melts and clumps when burned, the weave is 15 rows per cm, each thread is 2 strands of ca. 10 fibers per strand, and it is UV-positive.  Cotton, the weave is 14 rows per cm, each thread is 3 strands of ca. 8-10 fibers per strand, and it is a false UV-positive due to the use of a fabric finish. Poly-cotton mix, the weave is 14-15 rows per cm, each thread is 2 strands of ca. 10-15 fibers per strand, and it is UV-positive. Many of these modern replacement ribbons have a very straight (and stiff) horizontal weft weave and are almost never wavy. One unusual modern ribbon type that has been encountered recently is a plain white poly ribbon (clearly UV-positive) with the colors printed on - it looks fairly cheap and is readily identified.

My best advice is:  Buy only from a reputable dealer who specializes in Prussian &/or German medals and who has a good knowledge of ribbons.  Please visit my blog (http://blogs.ebay.com/tsa-li) and/or guides for further information.  Thank you for your visit!