As far as I know Alpaca is not a STERLING SILVER, Alpaca is a (Nickel, Copper, and /or Zinc alloy) that looks pretty much like Sterling .925 but it has lesser value than the sterling .925 does.
From the dictionary: (Sterling silver is an alloy having at least 92.5% of silver and the remainder 7.5% usually copper, the minimum millesimal fineness is .925).
Fine Silver 99.9% pure is generally too soft for producing large functional objects and in sterling silver is usually alloyed with copper to give strenght while preserving the ductility of the silver and a high precious content.
Other metals can replace the copper, these include Germanium, Zinc, Platinum as well as a variety of others additives including Silicon and Boron to increase resistance to tarnish.
This means that ALPACA has a lower percent of Silver and a higher percent of Copper, Nickel or something else, in others words when Silver contains an alloy of more than 7.5% of any other materials, it will become Alpaca.
Being said, Alpaca Jewelry won't be a precious metal but an interesting PIECE of ART, made by the Mexican artisans, especially made for Mexicans who CAN NOT AFFORD to buy Gold or Sterling Silver.
When you read on an Item ALPACA MEXICO it means that this item was made in Mexico using ALPACA, (Nickel) Alpaca is not a City from Mexico as someone has thought, however, when you read TAXCO MEXICO, it means that this piece of art was made in TAXCO. Taxco is a small city/town located on the Southside of Mexico City and belongs to the state of Guerrero, in Taxco Guerrero you'll find almost anything made of GOLD or SILVER, and of course you will find lots of ALPACA too, all of these to a wholesale price
I am not a master in this field and I am not intending to change your mind, I just wanted to share something that I know since I was little. I know that what it's known by Americans as NICKEL, Nickel Silver, Silver plated, etc., for MEXICANS it's called ALPACA, mexicans who can not afford to buy Gold or Sterling silver Jewelry, they use to wear ALPACA, as Alpaca looks like Sterling 0.925, it can be wear easily without being noticed, and that's why Alpaca Jewelry has to be Marked as Alpaca or anyone could be tricked.
If you go down to Mexico and try to impress a friend with a New Necklace, a Ring or Silver Jewelry, no matter that it's Sterling 0.925, you might be teased with the saying" That's Alpaca"
NICKEL SILVER is metal alloy of copper with Nickel and often but no always Zinc. It is named for its Silver appearance, and contains no elemental SILVER. Other common names for this alloy (nickel silver) are German silver, new silver and ALPACCA or ALPACA.
Silver Nickel first became popular as a based metal for silver-plated cutlery and other flatware or silverware. Nickel silver became widely used after 1840 with the development of electroplating as it formed an ideal and strong substrate for the plating proccess; it was also used for a cheaper grade of cutlery.
Alpaca or Nickel silver, has a wide variety of applications some Countries uses Alpaca for Kitchen utensils, coins, musical instruments, marine fittings and plumbing fixtures for its corrossion resistance and in heating coils for its high electrical conductivity, Taxco Mexico uses Alpaca to make low priced jewelry
In order to keep a safe trading, Artisans have to be honest Marking their items.
There are many marks printed on jewelry, YEAR, WHERE it was made, WHO made it and the MATERIAL it is made of. The most important thing is to look for the mark .925 or higher (.980) PRINTED on the item before buying it, Silver and Gold usually have these marks printed on them, if you don't see it, then you may buy it just because you like it.
GOLD has special marks too; 10kt, 14kt, 18kt or 24kt., if you don't see any of these, then it is NOT Gold but polished COPPER or BRASS or in the case of WHITE GOLD believe it or not, I 've seen someone on the streets selling polished STAINLESS STEEL instead. (kt) stands for Karat, a unit of measure of gold and (Ct) Carat, stands for Carat, a unit to measure Gems, Especially Diamonds.