I was kinda hoping (but not expecting) that it would be silicified (ie, composed of quartz) since quartz has a hardness of 7, and is therefore less scratchable than limestone. Limestone is comprised of calcite, which has a hardness of just 4, and is easily scratched by common items - such as a pocket knife. (My pocket knife was used for this test) Limestone also dissolves in weak acid. Because of the softness and the dissolution, calcite just doesn't seem like an ideal gemstone to me. However, I have to keep reminding myself that jewelry is exposed to much gentler conditions than outcrops and buildings! The pendant will certainly hold up in normal use for a very long time.
By the way, marble is metamorphosed limestone (or dolostone, a magnesium rich calcium carbonate). Marble is also completely comprised of calcite (and/or dolomite). The softness makes it a good sculptural medium, but the dissolution in weak acid means that anything exposed to the elements will be surficially eaten away in a relatively short time span. (natural waters and rain are usually slightly acidic, and even more so if it's being affected by industry as is the case with "acid rain")