**Question:** Specific gravity of enamel is:

**Core Concept:** Specific gravity is a measure of the density of a substance in comparison to an equal volume of water. In the context of dentistry, it is used to describe the density of enamel, which is the hard tissue that protects our teeth.

**Why the Correct Answer is Right:** Enamel is the hardest substance in the human body, primarily composed of hydroxyapatite crystals. Its specific gravity is approximately 1.9, which means it is denser than water. This density contributes to its strength and resistance to wear and decay, serving as a crucial component in protecting the tooth structure.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because specific gravity is not applicable to soft tissues like dentin or pulp, which are not mentioned here.
B. Although dentin has a denser composition compared to enamel, its specific gravity is approximately 1.55, far less than the correct answer.
C. This option is incorrect because it refers to the density of teeth rather than the specific gravity of enamel. Teeth are composed of both enamel and dentin, which affects their overall density but not specific gravity.
D. The specific gravity of enamel is not equal to that of dentin, as dentin has a lower density (1.55) compared to enamel (1.9).

**Clinical Pearl:** Understanding the specific gravity of enamel helps in appreciating its unique composition and role in tooth protection against wear and decay. Knowledge of these properties is crucial for dental professionals to diagnose and treat dental issues effectively.

**Correct Answer: D.** The specific gravity of enamel is approximately 1.9, making it denser compared to water (1) and other soft tissues in the oral cavity. This density contributes to the tooth's strength and resistance to wear and decay, which is essential for maintaining oral health.