To start with the answer, gems form in a variety of places from Earth's surface to the core-mantle boundary, so it is possible for gems found by mining to be present in ocean crust. Gems form through numerous processes, but they all require some form of liquid (either water, magma, or other super hot material) concentrated with dissolved elements or from pre-existing gems altered during metamorphism. There are igneous, sedimentary, and metamorphic gems.

Igneous gems typically form from magma cooling. Sedimentary gems usually form from water flowing through pore spaces or at Earth's surface. And metamorphic gems form from pre-existing material recrystallizing under both heat and pressure. As you mentioned, gems can be collected by miners, but some are discoverable at Earth's surface through various geologic processes (faulting and folding of Earth's crust, large scale crust uplift, and volcanism).

Some gems only form at specific temperatures and pressures deep below Earth's surface, like diamonds, and if they are not brought to Earth's surface at certain rates or under certain conditions, the gem is no longer stable and will change form.

Hi Lulu, great question! There are 5 basic requirements for crystals or gemstones to form: ingredients, temperature, pressure, time, and space. These gems can be created 4 different ways. Igneous gems are created deep within the earth. Hydrothermal gems are formed when mineral-rich water cools down. Metamorphic gems are ones that become gems due to super high heat and pressure (this is most common). Lastly, sedimentary gems form due to water depositing sediments.

Gems are formed from the constant cycling of rocks and minerals, a cycle that reaches from the inner parts of earth’s mantle to the mountains we see outside. Diamonds, sapphires, tanzanite, and rubies are mined from the deepest parts whereas opal and malachite are nearest the surface. To answer your question, yes. Since the sea floor is still part of the Earth’s crust, gems found on land can also be found deep in the ocean floor.

Miners normally dig to get gems or other minerals. There is no reason in theory why a mineral could not be exposed on the ocean floor as it could on land.

I expect that you're probably thinking about pearls. Pearls are made by clams, which are animals, and they are animals that live in water (although there are clams that live in rivers and lakes, not just in the ocean). This means that pearl isn't a mineral, and it certainly isn't mined.

Pearls are 'gems' that are only found in the ocean. I decided to search for diamond mining in the ocean. The ocean diamonds washed out of diamond mines on land: here.

"Millions of years ago, precious gem stones from the heart of southern Africa washed westward along the Orange River and emptied into the Atlantic Ocean."

The diamonds from the ocean are better:
Shikongo explains how nature ensured that only the "fittest" diamonds survived the journey along the Orange River, while weaker, imperfect stones were destroyed.

"Because the diamonds went through a high energy process, almost like a tumbling effect, only the best, high quality diamonds survived and made it to the sea," he says. As a result, Shikongo estimates that 95% of diamonds recovered from the sea are of "gem quality," compared to just 40-60% of diamonds from land operations.

Deep oceans can be mined, but not usually for gems. Ore deposits containing useful and valuable metals can often be found at deep sea hydrothermal vents, and these are sometimes mined. There is the interesting case of diamonds off the coast of Namibia. The diamonds were formed under the African continent and brought closer to the surface by a particular type of eruption known as a kimberlite eruption. The rocks that the diamonds were in eventually weathered away, and the diamonds were carried into the ocean by the Orange River. These diamonds tend to be high quality, because the more flawed ones tend to be broken up during transport. Some gem companies send out ships to scrape and vacuum the sea floor to collect the diamonds.

Both deep sea ore mining and seafloor diamond vacuuming can cause significant environmental damage, as they tend to destroy the habitats of creatures living on the sea floor. Mining on land also causes environmental problems, so it's probably best to conserve resources and get what minerals we do need from companies that do whatever type of mining they practice the most carefully.