Recently there have been TV reports on retail stores selling composite rubies. A composite ruby is not the same thing as a treated ruby. For generations rubies have been treated, usually with heat, to improve their color. This process is well known and accepted within the jewelry industry. If fact, heated rubies are so common that it's relatively safe to assume that any ruby you are considering purchasing has been heated. If it has not been heated there almost certainly will be a certificate from a gem lab indicating the stone is natural.
During heating there is often some level of minor glass residues found in surface-reaching cracks. This glass is the bi-product of the borax coating given the stone for its protection during the heating process.
Glass, or some other type of filling can be used specifically to fill in or minimize the appearance of surface reaching cracks in an otherwise beautiful ruby. If that treatment process has taken place the gem should be described as being glass filled. This filling process will also be noted on a lab report.
All these treatments are required by law to be disclosed by the seller. But these types of treatments should not be confused with a "composite ruby". Composite stones are created from multiple pieces of low-quality corundum which has been molded together with tinted glass. These composite stones have no place in the jewelry industry, and should not sold by any reputable jeweler.
For additional info on this topic there is a terrific article written by the AGA which describes ruby treatments and composite (junk) rubies. Link to that Ruby article here.