You are asking a logical and a rather insightful question.  Think about how you will want to use the silica powder you need.  Your answer will be dependent upon what you think the product will be used. 

For example, if you are grinding up glass bottles because you want an ultra fine product in order to make your own pigments for stoneware to be fired in a kiln.  Then your gonna want to begin your project with either clear glass or factory purchased silica powder.  A few steps later on in the production process.  You can then mix in the pigment(s) you want for your home made product.

If you are looking to use the silica powder to help thin the slag and to assist in the chemical processes at high temperatures to collect the impurity's during an assay, etc.,.  Then save your money for the many other things you will be needing to purchase or to fabricate.  Quartz sand, ground glass of any color, or quartz rock will do just fine for that kind of work. 

FYI -- For a few bucks.  You can also go over to your local big box hardware store and purchase a 100 pound bag of 30 or 100 mesh sand from the cement and brick aisle.

The active ingredient is the silicon dioxide, or SiO2.  Generally speaking, what is bottle or window pane glass?  It is silicon dioxide.  SiO2 comes in one color, clear.  It is the impurity's that give it any other colors.

In the refining processes, the Si does not like to chemically combine with your Au, Ag, or PMG precious metals.  So what you are really doing is asking the diatomic oxygen to reach out and grab other impurity's in the high temperature mix of an assay, etc.,.

Read up on the chemical property's of Pyrex glass before assuming to use it for its content of boron and oxygen.  It is the amount of alumina oxide used in the mix during the manufacturing process of Pyrex glassware which helps to give it a variety of useful functions and shapes in both the kitchen or the lab.

Your gonna also want to read up on boron and the natural mineral of boric acid found in products like the Borax brand.  In addition to being a fantastic natural cleaning agent for whites in the laundry.  It is highly hydroscopic, meaning it will add H2O molecule(s).  Sometimes a lot of additional water.  If you have ever used Borax or other brands straight out of the box around a torch.  Then you will know what I mean!  The water molecules will turn to steam and then "poof!" Up fly's the Borax out of your melting dish.  Nothing says, "I had no real clue in what I was just doing..." then the look on your face the first time that happens to you!!!  Its like a mini snow storm for your immediate work area.  Good thing you read the MSDS before you started... right?! 

If you want to make "borax glass" then look that one up and figure out how you would need to cook off the water molecules and then chip out, and then grind up your borax crystals.

Therefore, I would strongly suggest you read the (Material) Safety Data Sheets (MSDS) for any store bought product you choose to use.  Don't be afraid to ask questions and don't be afraid to do some extra research.  MSDS's can be found online with just a little effort.

As a middle aged law school student.  I know that recklessness in torts is having the "knowledge" and then to take an unjustifiable risk with that knowledge.  Is SiO2 dangerous to your health if you breathe it in?  Yes.  Various forms of lung diseases can occur.  However, if you have ever played in the sandbox as a child, ever been to the beach, or have you gone outside when the wind was blowing?  Then you have more likely than not, already breathed in some small amount of SiO2 into your lungs.  Some amount of risk is both unavoidable and always around us.  It is the recklessness or the ignorant taking of risk(s), aka the negligent taking of risks, that are at the root causes' of avoidable accidents and problems.

This is where I have a very deep respect for the old timers who had to figure all this chemistry out by the seat of their pants when it came to mining, assaying, and refining.  Maybe the old timers had no idea it was SiO2 that made the pour more runny.  Maybe they just observed that quartz ores were more runny than non-quartz ores during the assaying or refining processes?

The old timers paid for the knowledge we have at our finger tips today!  They paid for knowledge with their lives or the poor general health they had to suffer with for years before their time in the grave.  Silicosis is a very serious lung disease.  It is not to be taken lightly.  An excellent mining article on the subject was published in Aug, 2014, by Chris Ralph in the ICMJ's magazine.  It is titled, "Gold Ores of the Delamar District, Nevada."  Let's just say, there are very good reasons why modern rock drills and methods use water to manage the risks, and to keep the dust down!

Glass is not silica. If you need silica as opposed to a mixture of oxides, you must buy silica (SiO2).

Take a look at 'chemistry of glass' from Corning Museum of Glass:

https://www.cmog.org/article/chemistry-glass

Glasses begin as mixtures of oxides. Their compositions can be represented by listing the weight percentages of their components. Compare the percentages for 1, a typical, modern soda-lime-silica glass (used to make bottles and windows); 2, laboratory and some baking ware; 3, optical, high lead crystal; 4, 96% silica glass (can withstand very high temperatures); 5, a typical, ancient Roman soda-lime-silica glass.