Creo que debe ser "martellina"

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Note added at 18 mins (2019-05-18 15:24:47 GMT)
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o "martelina", según segun el contexto.

Coincido con Marcos en que debe der martelina, pero si es para picar me parece que se le dice brick hammer.

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Note added at 7 hrs (2019-05-18 22:14:28 GMT)
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A veces sirve buscar imágenes:

Martelina
https://www.google.com/search?q=martelina&rlz=1C1AVFB_enAR84...


Brick hammer
https://www.google.com/search?q=brick hammer&rlz=1C1AVFB_enA...

28 types of hammer
https://www.homestratosphere.com/types-of-hammers/

This is not a hammer at all. As Marco and Adolfo have both said, "marterina" is a variant of "martelina" and clearly refers to the same thing.

In Mexico, and I think elsewhere too, it is a tool with the basic shape of a hammer but with a different kind of head. Some martelinas have a pick at both ends of the head, some have a pick at one end and chisel or a mattock (adze) — that is, a horizontal cutting edge — at the other end, or a mattock at both ends, or a mattock at one end and an axe at the other. Here's a page which illustrates the various kinds: the main one featured is pico-pico, but there are also talacho-pico, talacho-talacho and talacho-hacha (talacho means mattock, of course):
http://www.elferretero.com.mx/Carrito/Producto.aspx?NumeroPr...

Here's another Mexican "martelina", very probably the kind they're referring to here:
https://www.masterdetector.com.mx/index.php?route=product/pr...

A stonemason's hammer (also known as a masonry hammer or brick hammer) has a flat hammer head at one end and a pick at the other, as Adolfo's images clearly show:

"A Stonemason's hammer, also known as a Brick Hammer, has one flat traditional face and a short or long chisel-shaped blade."
https://en.wikipedia.org/wiki/Stonemason's_hammer

But you won't find a "martelina" that has a flat hammer head at one end (at least, not in Mexico). It is a pickaxe, a pick mattock or a mattock. Given its function in your context (picar), I think pickaxe is most likely; it certainly can't be just a mattock. It's probably something like this; the image is just like most of those you find for "martelinas":

"Small pickaxe"
http://www.raz.ps/small-pickaxe-47-855

Or this:

"Outdoor Camping Fiberglass Handle Small Size Tools Pickaxe"
https://www.amazon.com/Outdoor-Camping-Fiberglass-Handle-Pic...

A pickaxe (also "pickax", or just "pick") is often a large tool used with both hands, but it can also be a hammer-sized tool used with one hand, as a "martelina" is. You could add "small" to make the difference clear, but I don't really think it's necessary.

"What is the difference between a mattock and a pickaxe?
Today, the terms 'pickaxe' and 'mattock' are often used to mean the same thing; they are however different tools.
A mattock has a broad adze (horizontal blade) usually with a pick or axe on the other end of the head.
A pickaxe has a pointed pick at one end of the head and a narrow chisel at the other."
https://www.wonkeedonkeetools.co.uk/mattocks/what-is-a-matto...

They could be referring to a pick mattock, which would do the job if you use the pick end, but I think just "pickaxe" would be best.