A Photographic Comparison of the Abrasiveness of Several Leading Toothpastes.
The problem with many common brand toothpastes is that they contain very large-sized abrasive particles that can quickly wear away tooth enamel. Because tooth enamel is not regenerated by the body, its loss is permanent. It is never replaced. As tooth enamel is lost, the softer dentin material underneath is exposed and thereby greatly enhances the risk of dental decay and tooth loss.
To preserve dental enamel, a toothpaste (dentifrice) should have extremely fine particle size—fine enough to not damage or abrade irreplaceable enamel, yet sufficiently strong to remove the biofilm of dental plaque.

This photo shows a popular toothpaste with particles so large that a single grain nearly fills the entire image area. Such huge particles are extremely corrosive to teeth, cleaning teeth while at the same time accelerating the erosion of enamel.

This photo shows the large angular structure of a leading whitening toothpaste whose sharp oversized structure whitens teeth by stripping away enamel with a sandpaper-like action.

This photo shows a popular household toothpaste with less angular but large stripping sizes with a rough, coarse, abrasive texture that can be highly damaging to dental structure. Long term use of such dentrifices can hasten the loss of enamel and eventually increase the risk of tooth loss.

Calcium carbonate (along with ultrafine particle zinc oxide shown below) forms the base of the most advanced toothpastes in the world. These ultrafine particle sizes allow for gentle removal of dental plaque with minimal erosion of tooth enamel.

Ultrafine particle zinc oxide (along with calcium carbonate in the preceding picture) forms the base of the most advanced toothpastes in the world. These ultrafine particle sizes allow for gentle removal of dental plaque with minimal erosion of tooth enamel.
The low abrasion Thieves Toothpaste fomulas contain the smallest particle-size commercially available calcium carbonate and zinc oxide.
All photos were made with a scanning electron microscope at 500 times magnification. Source: Essential Oils Desk Reference, 4th Edition, pp. 245-246