The difference between a ruby simulant and a synthetic is that one only looks like a ruby, and the other is a ruby from the chemical composition to the crystal structure (even if it is man-made).
Note that there is fraud with shady dealers offering rubies at extremely low prices. These are not rubies. They are synthetics and simulants with a lower value than what natural ruby prices command, and usually overpriced for what they are.
Ruby simulants are anything meant to look like a ruby that is not a ruby. This can mean assembled stones like two pieces of glass with red glue in the middle or other natural stones that show off a “ruby red” color like garnets (crystals that can form with rubies in nature).
Simulants offer ruby-red colors for a much lower price-point, though there are individuals who fraudulently sell them as the genuine article.
In comparison to ruby simulants, ruby synthetics are straightforward definition-wise. They are man-made rubies grown in a lab. There are also a variety of processes that can create rubies (and other assorted corundum). Based on the growth method, the synthetics can be grouped into two types of processes called melt and solution
The oldest one is the Verneuil process, named after Auguste Verneuil who created the first synthetic ruby (incidentally the first synthetic gemstone) in the late 1800s. Powdered chemical material is dropped through a flame to melt and fall to where it forms on a spinning pedestal. The result is a cone-shaped rough called a boule. The Verneuil process remains the cheapest and most common way of producing synthetic rubies.
The next method is the Czochralski process. It is similar to the Verneuil process, but instead of depositing material onto the base it is pulled away. A machine slowly pulls a synthetic corundum crystal from a solution to create synthetic rubies. The synthetic rough can appear similar to the Verneuil process, though it is often clearer and can be grown to larger sizes. There are further techniques that build on this process to produce super-clear and large boules.
Since the crystal material is spinning around for the melt processes, both processes can show curved striae and miniscule gas bubbles that can resemble pinpoints. These gas bubbles often form along curved striae, though they can have other strange appearances.
These features will never be present in natural rubies.
The flux growth version of synthetic rubies is much more expensive and time consuming versus the Verneuil and Czochralski methods. Flux-grown rubies can take more than a year to form and require expensive equipment to do so. The flux dissolves the corundum ingredients to form a chemical slurry that slowly cools and crystalizes, with the ruby crystals forming similarly to how they do naturally. As a result certain characteristics like fingerprints can also appear in these type of synthetic rubies
The last method is hydrothermal growth. Like the flux method, it is slow and requires costly equipment. Corundum ingredients are once again mixed into a solution, and undergo heat and pressure like in nature. Eventually the solution cools and becomes a crystal over time. This type of growth can also form fingerprints.
Due to how sophisticated some of these simulants and synthetics are, it is understandable how customer confidence can be shaken. This is why reputable gems dealers make it a point to carefully check and certify each ruby for authenticity and treatments.
Synthetic star rubies can be created as well. We already know how a synthetic ruby can be made, but the star can be added on with a specific heat-treatment and addition of certain elements to induce the star. Much like the beryllium diffusion method, the star is only surface-deep and can be lost with re-polishing.
Natural rubies can also be heat-treated to produce a star, if there is enough silk to re-crystallize and form a star. Otherwise chemical additives are used along with heat-treatment to synthetically create a star. In short it is possible for the stone to be natural, but the star to be synthetic.
These types of rubies also require advanced testing to confirm whether the star is natural or synthetic, since natural rubies can have rutile silk confined to shallow areas only around the surface of the stone. However, this case is incredibly rare as it would lead to a high-clarity star ruby.
As a gemologist I have only seen one natural star with this appearance, in a photo for a study by the lab I trained in; GIA (Gemological Institute of America). It was an orange star sapphire.
For many synthetic gemstone manufacturers the exact details of how they create their products is often held as a trade secret, much like a cook’s recipe for certain dishes. However, they will disclose the nature of their gemstones and provide documentation as needed. Some manufacturers also engrave the stones they make with microscopic ID numbers to help prevent fraud.