Dragons (like chickens, although they don't approve of the comparison) will occasionally swallow small rocks to aid in their digestion. In the wild when they hunted their food, and wolfed it down on the run, these would act kind of like rear molars (the Draconic term for them is often translated as "molars") and help grind up the more solid bits, like bone and armor, when in the stomach.
The harder rocks, like granite and marble, are preferred as the softer ones, like limestone and sandstone, tend to be broken down by the dragon's digestive system (sandstone is good for constipation, though). Sharp rocks, like obsidian and flint, are to be avoided as they can damage the lining of the Dragon's stomach (nevertheless, still a difficult task) and cause stomach upset. The ideal substance, of course, are gemstones. These are very hard, yet if worked, are quite smooth. Certain legendary figures are said to have had molars of diamonds and could eat the very earth itself.
The molars slowly dissolve and/or wear away over time. As small quantities of, effectively, powdered gemstone is released into the Dragon's system they are said to pass on certain benefits. There is much Dragon lore devoted to the moods, humors, and abilities brought on by what gemstones are in predominant influence within a dragon's system. Much of this is similar to human herb lore and there is great scholarly debate as to whether it has any basis in reality or is just social superstition.
In these civilized times Dragons seldom hunt on the wild nor need to devour their meals so quickly. Enjoying a wide variety of consumes, pates, and small bites, such things are seldom needed except by the old or infirm. Gem based molars are still rather fashionable amongst those who can afford them. The only time they do have any affect is in another form of draconic consumption: that of precious metals.
The natural color of a silver dragon is a dark, dusky gray. For gold a deep tan, and so on and so forth. But, by actually ingesting and adsorbing the metals their names correspond to, the metal circulates their system in solution, eventually being deposited in the scales. This not only increases their strength but also makes them glossy, shiny and healthy. In draconic social circles the glossiness of one's scales is seen as a direct correlation with the size of one's hoard (and thus status). Dragons who consume a considerable amount, though, are subject to tarnishing and require frequent polishing. This is often done by other servitor dragons with their rasping tongues. It is a great social embarrassment for a dragon to be seen with a tarnished coat. It occurs frequently in draconic folklore and colloquial speech. It implies that you are willing to spend large quantities of your horde on your looks, but not on maintaining them. Thus it often forms a cautionary tales with a moralism of balanced expenditure.
Very occasionally the true-forms of metals are ingested. Mithril for silver, Aurum for gold, etc. These are said to give that special extra gloss, imbue one with mystical qualities and freshen the breath. Often, though, these are just too hard to digest at all and pass completely through the system. As such this practice is most often derided as the highest form of conspicuous consumption.
Although they do not produce it themselves, charcoal is occasionally eaten by travelling Dragons. The taste of aromatic wood, such as hickory or fruit trees is preferred although mainly it is an experience of texture. They can aid the digestion slightly but are thought to provide no nutritional benefit whatsoever.