Someone may indeed varnish a shell, but this is not the usual way. Some kinds of shells are shiny because of the way the animal lives, but the sand isn't the polishing agent -- the animal itself is. When it is extended from its shell to crawl or feed, its body slides up around the shell like a cape; indeed, it's called a "mantle." Some mollusks cover their entire shell, others only a part. Since the mantle that covers the shell is also the part that actually manufactures shell, it can add tiny amounts of shelly substance as it enfolds the shell, smoothing and shining it, keeping its surface free of growths and encrustations, and from dulling by chemicals in the water.
The shells are not, as is often thought, painted or dyed. The colors are sometimes determined by heredity but in some species can also be influenced by the diet of the animal. Color in mollusks often serves as camouflage, but some pigments are primarily structural in function, serving to strengthen the shell. The yellows and reds of beta carotene are an example. Other colors, usually iridescent "mother-of-pearl" hues, are due to light refraction in combination with the actual structure of a translucent shell material.
The patterns and designs on shells are produced naturally by the animal that makes the shell, and the pattern is actually part of the shell, not a surface ornamentation. Each species has some patterns that are common to its members.
There are between 50,000 and 200,000, species of mollusks, depending on who's doing the counting. These figures are based on the number of species that have been described, and which of those the count accepts as valid, and the estimates of the number of undiscovered and undescribed species remaining on earth.
Mollusks' shapes are a product of heredity combined with habitat and life style for the most part. Shell shapes have evolved to make their lives easier. A snail that burrows through sand needs a shell that will move through wet sand easily -- best is a smooth and slender and gradually tapering shell, narrow end at the front, with no impeding projections, but mollusks have worked out many variations on this theme. A shell that needs lots of camouflage may have evolved a spiny or irregular surface which will catch and hold all sorts of camouflaging encrusting organisms. Spines are also helpful for discouraging predators and, in some arrangements, for life on mud...broad weight distribution.
The biggest marine snail is Syrinx aruanus, the Australian Trumpet, at 77.2 cm. (over 30"). The biggest American marine snail, the Horse Conch, Pleuroploca gigantea, is just over 2'. The world's biggest clam is of course the reknowned Giant Clam of the southwest Pacific, Tridacna gigas. The largest one on record, in the American Museum of Natural History, measures in at almost 55". (Incidentally, their favored diet is not divers but algae they farm within their own bodies.) These sizes are taken from Wagner and Abbott's World Size Records, published by American Malacologists and edited by Barbara Haviland of St. Petersburg, Florida.
You can hear the sound of the "ocean" not just with seashells, but also with a coffee cup or glass, or even by cupping your hand loosely over your ear. The usual explanation is that the seashell amplifies the sound of blood moving in your ear. Two pieces of evidence suggest that this is not correct. First, the sound is the same before and right after exercise (try running up and down stairs or doing jumping jacks), but it should be louder after you exercise since blood would be moving faster. Second, the sound is not drowned out by loud noise such as that heard next to a window mounted air conditioner. A better explanation is that the shell (or glass) acts as a resonating chamber, bouncing surrounding sounds back and forth, jumbling and amplifying them. This means that you should be able to hear the "ocean" better in a noisy room than in a very quiet one.
Taken from an article in the American Conchologist, March 1995 (p. 21), by Gary Rosenberg, Academy of Natural Sciences of Philadelphia