Blue gemstones such as agate sometimes show a blue color, but mostly light blue to medium blue. Any dark blue will likely be painted. Tanzanite is a faceted gemstone cut form transparent crystals.
The color varies from light blue to dark electric blue. It's not expensive with better reviews. "Lolite" is a faceted gemstone that is inexpensive, but exhibits a deep purple-blue color on well-cut stones. The color varies depending on the direction you are looking through the blue gemstones.
The blue type is called "Indicolite" and can be pale to very, very dark blue. Other blue stones that are considered gemstones or light blue to match ocean views from space include: topaz blue (made into navy blue), turquoise, aquamarine (pale blue), and zircon blue (heat treatment to make it blue perform).
Some "gemstones" such as sapphires may cost less than, say, lapis or spinel, which are gem quality. The price depends on the quality of the stone. In general, the quality level, the grade of gemstones such as diamonds, sapphires, and rubies are much more expensive than other stones.
Semi-precious stones can range from a few dollars to several hundred or thousands of dollars, depending on their quality, for example, "opal". Some semi-precious stones such as "agate" require a higher price for very beautiful stones but do not even compete with the better gemstones.